%% Image selected per Image Pickin' thread: http://tvtropes.org/pmwiki/posts.php?discussion=1358818439026200100
%% Please do not change or remove either without starting a new thread.
%%
[[quoteright:300:[[VideoGame/FableII http://static.tvtropes.org/pmwiki/pub/images/Fable_2_Cover_Art_No_text_5914.jpg]]]]

->''"He who fights monsters should see to it that he himself does not become a monster. And if you gaze for long into an abyss, the abyss gazes also into you."''
-->-- '''Creator/FriedrichNietzsche''', ''Beyond Good and Evil''
%%
%% One quote is sufficient. For additional entries, please use the quotes tab.

Usually, [[AntiVillain not quite a villain]], but they act antagonistically enough that [[AntiHero they're little better]]. Something has happened to our FallenHero: his [[DoomedHometown village was destroyed]], [[EverybodysDeadDave his friends killed]], [[AndYourLittleDogToo his puppy roasted on an open spit]], [[ArsonMurderAndJaywalking his bike stolen]], whatever. All that matters is that ItsPersonal, and he feels that the law just isn't suitable enough (or has become [[BadCopIncompetentCop too corrupt and ignorant]]) to be of any use to him in settling the matter. He may justify his actions by claiming that it's [[KnightTemplar Justice]] he's after, not vengeance, but anyone with half a brain can easily see that he's out for {{revenge}}... unfortunately, we can also see that the more he hunts the cause of his woes, [[FaceHeelTurn the more he takes on]] [[ThisIsYourBrainOnEvil the villain's personality and mannerisms]] -- something that our "[[{{Hypocrite}} hero]]" is too blinded by his single-minded goal to realize.

He may have [[WellIntentionedExtremist good intentions]] -- the fiend may well be too dangerous to be kept alive -- but ultimately, his [[AmbitionIsEvil obsession]] with meting out [[PayEvilUntoEvil due punishment]] ([[DisproportionateRetribution or worse]]) and his refusal to think about what he's doing twists him into a monster [[NotSoDifferent just as bad as]], or even worse than, the one he's hunting. And [[MoralEventHorizon even before he gets to that point]], it's nigh-impossible to turn him away; [[WhatTheHellHero calling him out on it]] will be ignored or [[ShutUpKirk retaliated against]]. ThePowerOfFriendship and ThePowerOfLove were lost to him the moment [[TheCallKnowsWhereYouLive the atrocity that sent him on his wild goose chase]] happened, and he feels that TeamSpirit is just a hindrance; [[HeelRealization Heel realizations]] will be [[IgnoredEpiphany ignored]]. Don't expect him to make a HeroicSacrifice or HeelFaceTurn anytime soon; if he dies in the process of bringing his nemesis down, it's usually with him crossing into Villainstown in his moment of glory. If he [[StartOfDarkness doesn't die...]]

The "fighting monsters" line represents what is a recognizable MoralEventHorizon for heroes, and both {{antihero}}es and {{Well Intentioned Extremist}}s live just near the boundary, especially the more pitiless GoodIsNotNice, KnightTemplar, [[TheUnfettered Unfettered]], PragmaticHero types. Engaging in VanHelsingHateCrimes is a good indicator of having ''crossed'' the boundary.

As expected, this twisted situation is very popular in the Revenge {{Tragedy}} genre, especially because of its inherent DramaticIrony [[note]]Nietzsche himself was also inspired by Ancient Greek tragic antihero archetypes; see also his first book ''The Birth of Tragedy''[[/note]]. This trope can also be used to demonstrate how "eye-for-an-eye" justice, while sounding like sweet [[CallItKarma Karmic]] EquivalentExchange Justice at first, can [[CycleOfRevenge easily spiral out]] into [[ForWantOfANail utter chaos]] if the hero lets his passions, wrath, and {{Pride}} forego rationality.

Not to be confused with CompleteMonster, although somebody can well become one by fighting such monsters. They can range from {{AntiHero}} to anywhere above. See also CycleOfRevenge, TheDarkSideWillMakeYouForget, ProtagonistJourneyToVillain, YouAreWhatYouHate, ThenLetMeBeEvil, and BecameTheirOwnAntithesis. If this trope happens to a child, it can be used as a FreudianExcuse. Compare AndThenJohnWasAZombie, where the character becomes a literal monster. IfYouKillHimYouWillBeJustLikeHim is pretty much a sped-up version of this. Political equivalents are ReignOfTerror, FullCircleRevolution, and MeetTheNewBoss. If the monster in question is an animal, that's AnimalNemesis. If the monster is supernatural, it's a decidedly ''un''heroic instance of TheHunter. Subtrope of SlowlySlippingIntoEvil.

Not to be confused with ''Manga/ThoseWhoHuntElves'', the '''fanfic''' ''Fanfic/HeWhoFightsMonsters'' (though this one is an example), [[IReadThatAs or]] ''VideoGame/MonsterHunter''.

[[noreallife]]
----
!!Examples
%%%
%%% Zero Context Example entries are not allowed on wiki pages. All such entries have been commented out.
%%% Add explanations to the entries before uncommenting them.
%%%

[[foldercontrol]]

[[folder:Anime and Manga]]
* In ''MahouSenseiNegima'', Haruna identifies [[spoiler:Kurt Godel]] as being [[http://www.mangafox.com/manga/mahou_sensei_negima/v30/c270/15.html one of these]]. He ''was'' an undeniable good guy as a kid, only to grow up into a completely manipulative {{jerkass}} little better than his enemy.
** In a flashback, Tsuruko uses the page quote as part of a lecture to a young Setsuna when showing her the [[EvilWeapon Youto Hina]].
* Ken, the pilot of Blade Gainer in ''{{Godannar}}'', is one of these. His obsession with killing the Mimetic Beast that killed his wife causes him to mercilessly hunt every Mimetic Beast down, without regard to how much property damage or civilian casualties he causes. [[spoiler:His last words before his death is for Lou to not become someone like him. Lou continues to fight off every single Mimetic Beast alone, but Ken's words prevent her from going down his brutal way.]]
* Scar of ''Manga/FullmetalAlchemist''. [[spoiler:His entire family is killed by alchemists and his brother gives up his life (and arm) to save him. Since his people have a quasi-religious reason to hate alchemy, he goes out hunting "In God's Name"]]. Especially ironic since he uses a form of alchemy to destroy his targets: state alchemists.
** A fact that he lampshaded himself when [[spoiler:he gave the Nina/dog chimera a release.]] Essentially, his thought process was "Well, I'm already cursed... may as well put it to good use."
** Comes up again when Colonel Mustang [[spoiler:tracks down Envy and burns him nearly to death]]. It takes Ed, Scar, and Hawkeye to [[spoiler:talk him down from dealing the final blow in a fit of vengeance. And then Envy, seeing how strong humanity can be, [[DrivenToSuicide kills himself]] anyway.]]
* ''{{Hellsing}}'' treats this trope in a very interesting way. Nothing is left to interpretation when Alucard shows clear disappointment and grief over [[spoiler:Father Anderson's decision to turn himself into a literal monster in order to remove Alucard from the world. He fails, but only closely.]]
* ''Anime/AfroSamurai''. Reread the first paragraph. That describes, in a nutshell, the entire first season of Afro Samurai.
* In ''Anime/KaleidoStar'', Yuri Killian is convinced that [[spoiler:his boss Kalos killed his father indirectly in order to achieve fame.]] During his long revenge scheme, he needs to win a competition in order to stay at Kaleido Stage and prove his stardom to Kalos. In doing so, he [[spoiler:is indirectly responsible for the death of another performer, Sophie Oswald.]]
* In the Meakashi chapter [[spoiler:and by extension the Watanagashi chapter]] of ''VisualNovel/HigurashiNoNakuKoroNi'', [[spoiler:Shion Sonozaki becomes convinced that her family kidnapped and/or murdered her LoveInterest, Satoshi Honjou,]] and starts on a chain of murders of her own to avenge him, culminating in [[spoiler:murdering his little sister when his very last request had been for Shion to protect her. This leads to a HeelRealization and IgnoredEpiphany.]]
* The monster hunters of ''{{Claymore}}'' use [[SuperpoweredEvilSide demonic power]] to increase their fighting strength. If they use too much, they transform and become living examples of this trope. Special note: here, it's more like "'''She''' who fights monsters".
** Usually it's by accident, though, [[SuperpowerMeltdown when they over-exceed their limit.]] However, there was one particular time when [[spoiler:Clare was willing to give up her humanity in order to kill Priscilla when she encountered her at last.]]
* ''Anime/CodeGeass'''s Lelouch is fully aware of and, in fact, ''embraces'' this trope. "I choose to commit evil in order to destroy the greater evil!" [[spoiler:At the very end of the series, he [[JumpedOffTheSlipperySlope jumps off the slippery slope]] ''on purpose'' in order to execute a GenghisGambit ''on the entire world''.]]
* ''Manga/{{Naruto}}'' is centered a lot around revenge, mainly on Sasuke's side. He wants to fight his [[spoiler:supposedly]] evil brother and therefore [[FaceHeelTurn joins the evil side]]. [[spoiler:After finally getting his revenge in an antiheroic sort of way, he goes off the deep end, and begins lashing out at Konoha, starting with abandoning his allies in order to get even with Danzo.]]
** Sasuke now seems to have [[spoiler:realized that he was [[HeelFaceTurn manipulated]] by [[BigBad Obito]] after he listened to the [[AssPull First Hokage's story]] about Madara and his dreams for the village. To make matters worse, this is complete with Karin being [[LoveMakesYouCrazy Ok]] with the fact he tried to kill her immediately afterwards.]]
** Strangely, Konoha itself. From how they treated Naruto in the beginning, it's pretty clear that they aren't the paragons of virtue. As a military government in a feudalistic world, they do pretty shady things to survive and keep peace, including [[spoiler:kill the Uchiha Clan after, at least according to [[UnreliableNarrator Obito]], blaming the Fox Invasion on them and outcasting them, prompting them to try a coup.]] Danzo, most of all, claims to want Peace by any means necessary, but his main goal is promoting Konoha's power across the world, with him on top, naturally. Danzo is also personally responsible for ''creating'' many of Konoha's greatest enemies, by ruining people's lives to the point of wanting revenge on Konoha in the name of the village's safety. [[spoiler:Itachi goes here as well, as he is willing to kill his family and go down in history as a murderer and psychopath just to keep the village safe AND protect Sasuke. Though this could be seen as a SadisticChoice given to him by Danzo.]]
** Hanzo of the Rain village is also implied to have fallen to this trope; in his youth, he was a WorthyOpponent of Mifune's who spared his life for fighting with honor and dreamed of uniting the ninja world; years later, he's a paranoid dictator whose endless warmongering has turned his country into a god-forsaken hellhole, and who crosses the MoralEventHorizon when he attacks Nagato and Yahiko's peaceful protest group out of fear that they'd try to usurp him. [[spoiler:This results in Yahiko and dozens of others dying horribly, Konan being scarred for life into an emotionless BrokenBird, and Nagato being crippled for life, going insane, and becoming the aforementioned Pain.]]
** Naruto himself almost fell into this trope; early Post-TimeSkip, his fury at the douchebaggery of Orochimaru and Akatsuki causes him to go [[SuperPoweredEvilSide Biju-mode]] more and more, each time faster as he continually calls upon the Ninetails. Kabuto even points out that Naruto wants to save Sasuke so much that he's willingly turning himself into a monster to do so. Luckily, Yamato sets Naruto straight and our hero becomes far less willing to transform until he can do so without losing his mind.
* In ''Manga/YuYuHakusho'', [[spoiler:Toguro's disciples were killed by a demon named Kairen, who challenges him to fight in the Dark Tournament. Toguro fights and kills Kairen, and wishes to become a demon himself (his appearance when using his demonic powers is somewhat similar to Kairen). However, this is partly due to [[MyGreatestFailure his guilt over his failure to protect his disciples]], and he wishes for an opponent strong enough to defeat him.]]
** Also, the StartOfDarkness for ''Manga/YuYuHakusho'' villain Sensui.
* In ''LastExile'', Alex Rowe skates dangerously close to this. Although he never makes the jump into full-out villain territory, his need for revenge against Maestro Delphine and the Guild is all-consuming, and at one point, he [[spoiler:orders his ship to open fire on Delphine's at the coronation ceremony of the new Empress, even though if they did shoot her down, the wreckage would have fallen and killed the new Empress (who is also the closest thing to a friend he has) and the combined leadership of both Anatoray and Disith, shattering the tenuous peace that had just now been established between them. Fortunately, his helmsman, Campbell, countermands his orders, advising the gunnery crews that 'the Captain's not himself'.]]
* NowAndThenHereAndThere: Poor Elamba. He was so devoted to killing King Hamdo he became as demented and twisted as he was.
* In ''Manga/{{Berserk}}'', Guts is every bit as cruel and sadistic as the Apostles he hunts at the beginning of the series. He deliberately tortures the Baron and Count in the beginning chapters, and, after fighting with Rosine, doesn't stop trying to kill her even after an innocent girl who was tagging along threw herself in front of him. Currently, the only thing preventing him from completely going off the deep end is [[MoralityChain his need to protect Casca from the monsters he fights]], and, occasionally, [[SuperpoweredEvilSide from himself]].
** And the worst part of this all? If he ''wasn't'' willing to sink to that level of depravity, he would have gotten killed a dozen times over. As bad as he is, ''everything else is '''{{wor|ldHalfEmpty}}se'''''.
** As if that weren't enough, due to his unstable personality, which developed in the face of [[DeusAngstMachina unreasonable adversity]], coupled with the effects of this trope, Guts now struggles with a rather powerful [[EnemyWithin inner demon]].
** Guts' Dragonslayer was originally "just" a huge sword that looked more like a slab of iron. Over the course of the series, the Dragonslayer has slain so many Apostles that it has become demonic as well.
* Ray Lundgren from ''GunXSword''. At least the protagonist Van, while skirting this line, still had standards about not ruining property, taking innocent lives, etc. Ray, on the other hand, will gladly kill ''anyone'', innocents included, if they stand between him and The Claw. He even claims that, as long as he can kill The Claw, [[AndThenJohnWasAZombie he doesn't mind becoming a murderer himself.]]
** Subverted when he [[spoiler:wastes his only chance of killing his nemesis by [[ExactlyWhatIAimedAt purposefully missing his last shot]] in order to foil The Claw's plan once and for all]].
* Howl, from ''Anime/HowlsMovingCastle'' (the movie, not the book), is [[spoiler:becoming a monster in his efforts to combat both sides of the war. It's outright stated that only Sophie breaking his contract with Calcifer will prevent this]]. Considering her methods, Suliman might qualify, too.
* Used in ''Manga/OnePiece'', where one of the {{Big Bad}}s, Arlong and his crew of fishmen, oppress humans with the SocialDarwinist explanation that since they are physically more powerful than humans, they deserve to rule over them. Flip ahead several hundred chapters and it's revealed that they behave this way [[FreudianExcuse because they were once horrendously oppressed and enslaved themselves]] by humans that [[FantasticRacism considered them subhuman and treated them accordingly]].
** Akainu in particular needs to be mentioned. His very first appearance is in a 20 year flashback where he destroys a massive ship of innocent refugees because there might, just ''might'' have been one of his targets on board. For God's sake, even ''Blackbeard'' has more empathy than this man.
** Not just him, but the World Government as a whole. Shortly after Gold Roger was executed, they found out that [[spoiler:he had a child with an unknown young woman]]. Their solution? [[spoiler:hunt every young mother or soon-to-be-mother they believed to be suspect, and kill them and their children]]. Pretty damn cruel. Not to mention [[spoiler:it didn't even work anyway: the young woman who actually was pregnant with Roger's child held off on giving birth for ''20 months'', until the hunt had stopped, leading to her DeathByChildbirth]].
* This trope is given a bit of a workout in ''Anime/{{Monster}}'': [[spoiler:Johan's antagonizing of Tenma and Nina is, in many ways, because he wants them to chase him and make this trope come into effect. In some ways, he actually succeeded with Nina, who shot him during the events of the first episode; however, he utterly fails to break Tenma.]]
* In ''LightNovel/ShakuganNoShana'', all flame Hazes aside from the titular character have some elements of this, and it is stated that most of them became flames because they have a personal hatred of the Crimson Denizens.
** It's the clearest with Margery Daw when she is first introduced trying to kill a Denizen that she knows is not only [[spoiler:restricting his harvesting to protect the balance among {{Friendly Neighborhood Vampire}}s and humans]], but is also under Shana's protection, thus resulting in fighting someone who should be her ally.
* Char Aznable of ''Anime/MobileSuitGundam'' fits this nicely, up until he acknowledges to his sister that revenge wasn't the best idea later on in the series. However, he turns back to being this in ''CharsCounterattack''.
** In {{Gundam 00}}, [[spoiler:Louise Halevy]] becomes this as [[spoiler:she turns into a DarkActionGirl to get revenge for her family's death and the loss of her hand. She ''does'' get to kill the girl who caused her disgrace, but by that point, she's so far gone that her ex-boyfriend, Saji, has to dive over the DespairEventHorizon to stop her from hurting herself any more.]]
*** [[spoiler:Speaking of murdering the killer, Setsuna F. Seiei almost crossed this point of no return, too, by trying to kill Ali Al-Saachez, the man who fooled him into murdering his own parents in the name of God. Thank to Marina Ismail's spiritual intervention with CrowningMusicOfAwesome by Kenji Kawai, he was saved from becoming a monster like Saachez in time.]]
*** In fact, Celestial Being as a whole qualifies for this trope in the first season.
** The same thing happens in ''GundamAge'', [[TearJerker heartbreakingly.]] [[spoiler:Flit has been a [[MessianicArchetype Messiah]] who worked to make humans understand each other while defending themselves against the Unknown Enemies -- until his love Yurin dies in the hands of the child Saachez clone, Desil Galette, at which point Flit lost all his idealism and became a murderous monster just like the UE. It got worse after Yark Dole reveals that the UE are HumanAllAlong, as Flit refuses to recognize them as fellow humans who are also suffering in the hands of the evil Earth Federation.]]
* ''Manga/DeathNote'':
** Light Yagami begins using the supernatural notebook to rid society of criminals, but soon his black list expands to include anyone who stands in his way for any reason, starting with the FBI, and he even declares that he'll start executing people just for being lazy. Along the way, he coolly manipulates the feelings of both people and shinigami. Repeatedly stating that he plans to become the god of the new world he is trying to create doesn't help matters, either. Declaring that he will eventually execute people for being lazy implies that Light has done away with the slippery slope completely and simply jumped off the MoralEventHorizon. Ryuk even lampshades this, but he's just in for the spectacle.
** Teru Mikami uses the notebook to eliminate minor and reformed criminals.
* The titular character of ''ChirinNoSuzu'', motivated by the senseless killing of his mother by the Wolf King, goes on to become just as ruthless and bloodthirsty as the wolf.
* ''Manga/{{Bleach}}'' gives us Kaname Tousen, who joins the BigBad because he wants to seek "justice." [[spoiler:Turns out all he really wanted was revenge]].
* In ''Anime/TheTowerOfDruaga'', whoever reaches the top of the tower, defeats the monster Druaga, and gains the [[MacGuffin Blue Crystal Rod]], by the very nature of the tower, is in danger of becoming Druaga's next incarnation. Fortunately, how fast this happens also depends on the person's nature. FallenHero Gilgamesh held out for a century before making his FaceHeelTurn, and then, he retained his humanity. [[spoiler:Neeba, however, made his FaceHeelTurn BEFORE he got the rod and wasted no time going OneWingedAngel.]]
* The ''El Baile De La Muerte'' arc of ''Manga/BlackLagoon'' is an exploration of this trope in regards to [[NinjaMaid Roberta]], who sets out on a RoaringRampageOfRevenge against the people responsible for the kaboom that killed her master, a rampage that has her going AxCrazy during the course of it. [[spoiler:It turns out that Caxton, the guy who ordered the attack, is '''not''' the monster that Roberta believes he is, but someone whose well intentions resulted in innocent people getting hurt because he and his military are locked in the eponymous "dance of death", using violence as their main means to deal with violence. Only Garcia, Fabiola, and a {{plan}} by Rock are enough to snap Roberta out of psycho-mode so that she can return home to those she loves.]]
* In ''Anime/YuGiOhGX'', Judai takes revenge on the Dark World monsters for Johan's supposed death, and ultimately embraces the concept of being evil in order to fight evil, and then proceeds to burn entire villages to the ground, while part of him sits trapped in his own mind, muttering ItsAllMyFault. [[spoiler: It is. If he had not rushed into Brron's castle without O'Brien and Jim, everything would've turned out differently. Well, if he also failed to be so supremely focused on "I'll do absolutely anything to get my revenge, no matter who or what I have to sacrifice." anyways.]]
* Aporia is like this in ''Anime/YuGiOh5Ds''. After surviving the BadFuture where humanity was wiped out by the Meklord Emperors, he tried to SetRightWhatOnceWentWrong but barely cared about how many innocents had to die in the process, becoming as much a machine as the things that had caused him so much grief and using the Meklords himself in his goal.
* Even after his semi-HeelFaceTurn, Accelerator in ''LightNovel/ACertainMagicalIndex'' will mercilessly kill anyone who harms innocents, or even worse, try to harm [[spoiler:Last Order or any of the Misaka clones]]. Granted, he calls himself a villain despite saving innocents, since he knows his rather violent methods are not "heroic", especially compared to Touma, the one Accelerator considers fits "the hero" type.
* In his first appearance in ''KirbyRightBackAtYa'', Knuckle Joe was consumed by hatred in his quest to get revenge on the Star Warrior who killed his dad. This led him to do terrible things, such as trying to kill poor Kirby who wouldn't even fight back. Then cue the appearance of Meta Knight, who reveals that he was the one who killed his father and [[YouMonster calls him a Demon Beast/monster for the awful acts he committed, as well as his abandoning of reason]]. This was the perfect opportunity for Nightmare, who later turned Joe into a monster that could fire spikes at the cute pink spud. See? Not all monsters are made by Nightmare himself - it's also possible to become one by abandoning reason rather than listen to it as well as living only by hatred.
* Very often in the subtext in ''GhostInTheShell''. Even though he lacks cybernetic enhancements and has no special forces background, Togusa is a very valuable member of the team, as with his former career as a street cop and a wife and daughter at home, he often serves as the voice of reason and urges the others to restrain themselves from using excessive violence and illegal methods.
* This happens to [[spoiler: Sayaka]] in Anime/PuellaMagiMadokaMagica, though part of it is due to her despair at slowly [[spoiler: losing the boy she's in love with]], and being powerless to stop it. As she fights more and more witches, she starts to become blood-lustful, reaching the point where she seems more insane than the witches, [[spoiler: even before she turns into one herself]].
** [[spoiler: This is the fate of ''every'' Puella Magi. If their [[SoulJar Soul Gem]] ever becomes fully [[TheCorruption corrupted]], they are doomed to become witches themselves. The final episode has Madoka find a way to stop this cycle, at the cost of her existence.]]
** In the [[Anime/PuellaMagiMadokaMagicaTheMovieRebellion Rebellion Story]] [[spoiler: Homura, who had spent many timelines fighting Kyubey to prevent Madoka's contract, ends up hijacking his plan (and literally becoming a Luciferlike figure) to circumvent the Law of Cycles Madoka created.]]
* MutekiKanbanMusume: When Megumi accuses Miki (a prototype of TheBully seen on fiction, incredible strong and dumb) of bullying her, Megumi is truly surprised when Miki retorts that Megumi is the true bully – by using MaliciousSlander, sabotaging Miki’s work and setting tramps against Miki – a realistic depiction of TheBully in RealLife.
* In ''Manga/FairyTail'', it's been stated that those who use too much dragon slayer magic take the risk of becoming a dragon themselves.
* Kurapika from ''{{Hunter x Hunter}}'' seems to be turning into this...
** [[DarkerAndEdgier More like Gon....]]
* Ian deals with this in ''Manga/ACruelGodReigns''. He struggles with being angered at Jeremy's failed attempts to recover from being a {{Hooker with a Heart of Gold}} and {{Functional Addict}} and falling in love with him, but he worries that if he acts upon either of these, he will become like his {{Archnemesis Dad}} who caused all of Jeremy's problems in the first place.
* Yuu from ''Manga/{{Holyland}}'' fears that his taking the fight to delinquents and gangsters as a BullyHunter makes him little better. Various characters try to assure him that it doesn't.
* Discussed extensively within ''Manga/AttackOnTitan'', both by the narrative itself ''and'' the characters. Leaders such as Erwin Smith and Dot Pixis embody the belief that throwing away human compassion is necessary to win against the Titans, ruthlessly sacrificing the lives of their soldiers for even the smallest chance of victory. Armin comes to believe that throwing away one's humanity is the only way to win against monsters, while Jean expresses to him doubt over whether such methods are truly worth it. Mikasa has honed herself into a ruthless fighter for the sake of Eren, even going so far as to threaten the lives of her own comrades. On the other hand, Eren is a far more literal example: [[spoiler: becoming a Titan in order to fight against the Titans]], but also struggling to embrace the ruthless actions others ask of him. Finally, there are the other [[spoiler: Titan Shifters]], who play with this trope in an interesting fashion: [[spoiler: their own humanity comes back to haunt them, causing them to make critical mistakes. Annie spares Armin, and finds her mercy repaid with being exposed and trapped by the military. Reiner is unable to cope with the guilt of his actions, suppressing his memories and creating a false persona in order to function. Bertolt internalizes considerable remorse and self-loathing, unable to act on his own even to prevent Reiner's SanitySlippage and when his own emotions boil over, he makes reckless mistakes that allow Eren to be rescued. Finally, Ymir fell InLoveWithTheMark, and is successfully blackmailed using her feelings for Krista]].
* Rock in ''Manga/BlackLagoon''. Throughout the series but particularly during Roberta's Blood Trail where Rock [[spoiler: develops a sadistic side due to Revy and Roanapur's environment, manipulating major players in the city and gambling with peoples lives including Garcia, a mere child. Fabiola even states that Rock is now considered by her to be "the biggest number 1 bastard in Roanapur".]]
[[/folder]]

[[folder:Comics]]
* Franchise/{{Batman}}'s greatest fear is that he will become this, if he hasn't already. In fact, this is the way many other heroes see him, and they are not entirely wrong ([[DependingOnTheWriter depending on who's writing him]]).
** It's also why Batman so strictly adheres to ThouShaltNotKill: having that as a [[TheFettered line that he]] ''[[TheFettered never]]'' [[TheFettered crosses]] is a [[JumpingOffTheSlipperySlope barrier to slipping over the edge]] and becoming as much of a monster as the psychos he fights. Out of all his enemies, the Joker manages to be the one who makes him come very, '''very''' close to breaking his one rule...and that's because the Joker goes out of his way to make him break it.
*** Batman has had to be restrained more than once from killing the Joker in a few stories, like the Hush storyline when he thought the clown had murdered a childhood friend of his.
** [[AntiVillain Mr Freeze]] perfectly represents this fear and this trope in the animated series with the gusto he goes after those responsible for what happened to his wife, Nora. Freeze wants vengance at any and all costs.
*** The cycle continued in ''WebAnimation/GothamGirls''- Nora's sister Dora Smithy became obsessed with vengeance against ''Freeze'' for what ''he'' did to Nora, and in the process became more or less his female double [[spoiler:a fact cemented by Dora's own KarmicTransformation into an icy mutate with no emotion left but her love for Nora, packed away in a cell in Arkham just like her hated brother-in-law]].
* ComicBook/ThePunisher from Creator/MarvelComics is often presented this way whenever he makes a guest appearance in more idealistic books like ''Comicbook/{{Spider-Man}}'' or ''Comicbook/{{Daredevil}}''. However, in his own books, he's portrayed as a profoundly messed up individual, more tortured machine than man.
* The Vigilante, Adrian Chase, slew himself for this very reason.
* ''The Lone Wolf'', in Mike Barry's novels, ended up going so out of control that his own sidekick took him out. Mike Barry, actually [[SueDonym Barry Malzberg]], felt pleased to bring the series to this conclusion.
** And yet, he still worries. After 'The Slaver' storyline, he is troubled by the graphic extremes he went to rescue innocents and dispose of the bad guys. He just finished shooting some people in the forehead...
* In ''Comicbook/{{X-Men}}'' comics and especially [[Film/{{X-Men}} the movie trilogy]], {{Magneto}} - a survivor of the Holocaust - [[WellIntentionedExtremist is so determined to ensure that what happened to him never happens to his fellow mutants]] that he becomes increasingly xenophobic and genocidal towards unpowered humans, quite happy to wipe them out in order to ensure mutantkind's supremacy, and ultimately winds up little better than those who prompted him to begin his fight.
** A fact that former Nazi Red Skull pointed out. [[ArsonMurderAndAdmiration It should be worth noting, though, that Red Skull was sincerely trying to praise Magneto.]] [[YourApprovalFillsMeWithShame He somehow didn't know that he was a Holocaust survivor.]]
* Rorschach from ''ComicBook/{{Watchmen}}'' describes this in great detail, recounting how he became a dark and gritty AntiHero first (though he had a violent childhood) by brutally taking out his anger and disgust on a [[spoiler:kidnapper who had butchered and fed a little girl to his dogs]] by [[spoiler:setting him on fire.]] It even affects Rorschach's psychologist. The chapter in which we learn this is even called "The Abyss Gazes Also" and ends with the Nietzsche quote above.
** Indeed, this trope also features in the ''Tales of the Black Freighter'' sub-comic: a lone, marooned sailor, convinced the the titular ship will raze his village in his absence, returns to defend his loved ones on a raft of his mates's bloated corpses. He begins his bloody crusade against the raiders -- [[spoiler: except the raiders hadn't arrived yet. He ends up attacking his wife and, horrified at what he's done, throws himself into the ocean, [[UnwittingPawn where the freighter collects his condemned soul.]]]]
** Ozymandias could count toward this as well, seeing as [[spoiler: his solution to keep Russia and America from wiping each other out with nuclear strikes was to kill three million people before the missiles could be launched. Veidt strongly hints in his last conversation with Jon Osterman that he has nightmares of being taken into a ghost ship to be surrounded by murderers, in exactly the same manner as the ending to ''The Black Freighter.'']]
* General "Thunderbolt" Ross from ''Comicbook/IncredibleHulk''. This is made especially clear in ''Hulk: Gray'', where many parallels between Ross and the Hulk are drawn and Ross grows more and more fanatical in his pursuit of the Hulk as time goes on. Eventually, in his pursuit to defeat him, he became what he hunted: [[spoiler: a Hulk]]. He even lampshades it.
* ''ComicBook/{{Blade}}'' pretty much constantly struggles with this. In fact, the storyline Midnight Massacre in Midnight Sons, where Blade's hatred of the occult led him to accept the power of a spirit in exchange for the ability to exterminate all supernatural life made him rethink just how he goes about his mission.
* ComicBook/SpiderMan villain Supercharger was the son of a scientist who was obsessed with mapping the biology of superheroes and was given electric powers in the very accident that killed his father. So, the guy was embittered against superheroes and felt that they were ultimately more trouble than they were worth. Somewhat understandable. So he demonstrates that people with superpowers are dangerous by going completely crazy with his powers so people will see how dangerous he is.
* The Jedi Covenant from the ''Comicbook/KnightsOfTheOldRepublic'' comics becomes so determined to stop the Sith from re-emerging that they're willing to kill ''their own apprentices''. [[spoiler: Ironically, the Covenant turns out to be puppeted by a Sith Acolyte who makes up for his lack of power by being a borderline MagnificentBastard. When the leader of the Covenant finds out about this, he does a VillainousBreakdown, VillainousBSOD, and HeelFaceTurn, in that order]].
* ''ElfQuest'' has the Go-Backs as an example of a whole culture falling prey to this. All they wanted was to follow the call of the souls of their ancestors to the Palace of the High Ones, but the Frozen Mountain Trolls fought a war to keep them away from it. The Go-Backs didn't only grow into ruthless warriors, but also took up a habit of their enemies that disgusted even other trolls: eating the bodies of the enemies they killed in battle.
* The [[Characters/GLRedLanternCorps Red Lanterns]], especially [[Characters/GLGreenLanternCorps Laira]], who was the first ComicBook/GreenLantern to use the newly-written premise to kill for revenge, which was why she was expelled from the Green Lantern Corps. Feeling betrayed, she became a perfect host for a Red Lantern Ring, which then turned her into as dangerous and murderous a psychopath as many of the [[Characters/GLSinestroCorps Sinestro Corps]] members that she hated so much.
** {{Sinestro}} is this even more. To bring order to his planet, he turned it into a totalitarian regime, becoming the symbol of tyranny in a star system. Once Hal Jordan took him down, he decided that Guardians are not up to the job of bringing order and defeating crime in the Universe and swore to destroy the Green Lantern Corps and replace it with his own order. He has no problems teaming up with several DCUniverse villains and, in the end, created his own corps, openly acknowledging that most of its members are of the same kind as those he once fought. [[spoiler: When his daughter, Soranik Natu, had to become ruler of their native planet, Sinestro was more than interested if she is gonna follow the same path as him]].
** Guardians tried so hard to ensure GLC victory in the war with the other Corps that they established several oppressive rules, allowing Corps members to kill and banning relationships between the members. They had no problems with giving Larfleeze a star system and giving him the location of the Blue Lanterns' (who wanted to become their allies) headquarters to make him stay away from rest of the Universe.
* Rick Grimes from ''ComicBook/TheWalkingDead'' is becoming more like this as the series progresses. His actions have become increasingly brutal as his obsession with keeping his son and the group safe grows. He himself is beginning to recognize this however, although we'll have to wait and see if he can pull himself back from the brink.
* Megatron in the [[TheTransformersIDW IDW comic books]] was this. He initially formed the Decepticons to try bringing equality and justice to Cybertron, the Cybertronian society having become so corrupt and fascistic that Autobot thugs freely handed out beatings to innocent Cybertronians for no reason and the Autobot senate's answer to dealing with peaceful protests was to have all of the protestors shot. However, over the course of four millenia of war, as well as the brutal things he did to gain power in the first place, he's become exactly the kind of heartless and vicious tyrant that he originally despised.
** [[spoiler:Amazingly, he eventually had a MyGodWhatHaveIDone-moment, and pulled a HeelFaceTurn. And while there are some on both sides who acknowledge he's changed, there are a lot more who refuse to accept it.]]
* While Comicbook/{{Superman}} rarely comes close to this trope, he is very aware of it and {{lampshade|Hanging}}s it on several occasions, mostly in response to {{Anti Hero}}es who are just as bad as the villains they fight. He even occasionally criticizes his own actions in the newspapers as Clark Kent when he does veer dangerously close to this territory as Superman.
** This fear is what made him give a piece of kryptonite to Batman.
** In ComicBook/ForTheManWhoHasEverything he [[spoiler: Nearly beats Mongul to death]] but was distracted.
* Reed Richards of the Comicbook/FantasticFour blatantly averts this trope when dealing with his ArchEnemy, DoctorDoom. To this day, he still doesn't outright hate Doom, no matter what horrible things Doom inflicts upon him. In fact, in one storyline where he was exceptionally pissed off, Reed told himself that he really had to kill Doom this time, only to break down in grief and pity when he confronted him, and wound up pleading with Doom to reform.
* In ''ComicBook/KingdomCome'', Orion has killed {{Darkseid}} and became ruler of Apokolips. In order to maintain peace, he has to make many questionable choices and Superman notes that he is now more like his father than ever before.
** It only skirts this if you assume that he isn't trying to justify his becoming this and was telling the truth. Orion states that he tried to make Apokolips a better place. The residents couldn't handle freedom. This forced Orion to become more like Darkseid in order to keep the population under control.
* In ''Harry Kipling (Deceased)'', the [[FlatEarthAtheist New Atheist Militia]] is inspired by Kipling's example and sets about killing gods - by committing genocide [[GodsNeedPrayerBadly against their followers]]. In fact, their denial is so intense and fanatical that it manifests as what Kipling describes as an 'anti-god', and the NAM is as trapped by their own anti-god as the believers are by their deities.
* The fact that Crux turned himself into exactly the type of monster he's dedicated his life to hunting is called attention to in Issue #5 of ''ComicBook/RedHoodAndTheOutlaws''.
[[/folder]]

[[folder:Fan Works]]
* ''FanFic/HarryPotterAndTheMethodsOFRationality'' plays with this trope. As early as the Sorting Hat, people have known that Harry could be a terrible force for evil if he turned. Dumbledore even considers an alternate interpretation of the prophecy, that the Dark Lord shall mark him as his equal, to be that Harry will become his successor. Meanwhile, Quirell himself seems to be actively attempting to use this trope on Harry. Even Harry is worried about it, given how far he's strayed to the Slytherin side of things since the beginning of the story, and how he refers to Hermione, Neville, and [=McGonagall=] as his consciences. However, while he fights with Slytherin tactics, and even declares early on his desire to become a god and rule the world, he and Voldemort have very deeply different goals, and Harry is disgusted by the level of cruelty displayed by Voldemort, and has never once displayed the same cruelty.
** It's implied that this trope [[BreakTheCutie happened to Dumbledore]] and while he never became Voldemort, and still doesn't understand why Voldemort did the things he did, fighting Grindlewald brought him so close to the abyss that fighting Voldemort made him break several rules and do many things he regretted.
* In ''FanFic/AeonNatumEngel'', a Nazzadi (artificially created human-like race created for the sole purpose of annihilating humanity, but rebelled) is supervising a project of Replica soldiers (artificially created soldiers for sole purpose of annihilating the enemies of humanity, including the Nazzadi's creators, the Migou, and they have no concept of rebellion).
* ''Fanfic/DoomRepercussionsOfEvil''
-->"No! I must kill the demons" he shouted. The radio said "No, John. You are the demons". [[spoiler:{{And then John was a zombie}}.]]
* One criticism of ''FanFic/TheOpenDoor'' is that while the Necrons are {{Omnicidal Maniac}}s, the {{Unscrupulous Hero}}es[=/=]{{Villain Protagonist}}s fighting them are themselves highly culturally and militarily aggressive. Their aspiration to and failure to achieve TheUnfettered status only serves to blacken their image further [[BaseBreaker in the eyes of some.]]
* A central theme in ''FanFic/AvatarTheLastAirbenderRevised'' is preventing oneself from succumbing to He Who Fights Monsters by only killing when absolute necessary. This is present primarily in Katara's character arc, although it fits into those of others as well. A paraphrase of Nietzsche's quote is used as the ArcWords for the first three books to reflect this.
* Not only the theme but the name of ''Fanfic/HeWhoFightsMonsters'', a RosarioToVampire story where Tsukune never meets Moka or the others. To keep anyone from finding out that he's human (and thus stay alive), he's eaten bugs, made several [[ImprovisedWeapon Improvised Weapons]], and [[spoiler: killed 5 students, ''including Inner Moka'']]. "When your life's on the line, there's nothing that you won't do to survive."
* ''FanFic/TheImmortalGame'': After Twilight Sparkle is freed from [[SuperpoweredEvilSide Nihilus']] control [[spoiler: and her mind [[SplitPersonality splits in two]]]], she [[spoiler:, or rather, her dominant personality, Sparkle,]] becomes an ActualPacifist, as she fears that this trope will come into play -- she was [[AndIMustScream forced to watch]] Nihilus perform countless acts of violence and torture ForTheEvulz, and is afraid that if she starts fighting those responsible for her transformation, [[BloodKnight she'll start enjoying it]]. [[spoiler: When [[BigBad Titan]]'s torture causes her mind to [[NiceJobFixingItVillain fuse back together]], she realizes she was being foolish -- she is ''nothing'' like Nihilus, and she will ''never'' let herself sink so low. Cue asskicking.]]
* In ''FanFic/{{Respect}}'', [[spoiler:this happens to [[SmilePrettyCure Yayoi]] [[ShrinkingViolet Kise]]. She makes a contract with [[Anime/PuellaMagiMadokaMagica Kyubey]] to stop bullies from tormenting her... and ends as a berserk supervillain, responsible for turning her home into a GhostTown.]]
* In ''FanFic/LowLight'' L realizes he's become this when he [[spoiler: kills Misa with the Death Note.]]
* The ''Fanfic/PonyPOVSeries'' has the Mane Six in one of the alternate universes seen in the GateOfTruth. In this universe, they've become KnightTemplar dictators of the world using the Elements to 'correct' anyone they see as disharmonic. It gets so bad [[spoiler:Cadence goes [[SuperPoweredEvilSide Nightmare]] [[GodzillaThreshold as a last resort]] to stop them.]]
** In [[BadFuture Dark World]], we have [[spoiler:the Nameless Passenger, AKA, Nightmare Paradox. A potential future version and SuperpoweredEvilSide of Twilight, she went Nightmare to get revenge on Discord for his crimes, but ended up turning the Dark World into a GroundhogDayLoop and erasing it and everyone in it over and over again for the sake of punishing Discord. Even after Discord had a HeelRealization, she ''forces'' him to remain an EvilOverlord so she can keep punishing him. Most notably, she makes it a point that his [[DaddysLittleVillain daughter Fluttercruel]] ''always'' dies because [[EvenEvilHasLovedOnes she's the only reason he still tries anymore]]. It's been pointed out inuniverse she's become NotSoDifferent than Discord himself, which is one of her {{Berserk Button}}s and the characters consider her even ''worse'' than him.]]
* Discussed in Fanfic/RainbowDoubleDashsLunaverse. Luna doesn't dare take drastic action to curb the rampancy of the [[DeadlyDecadentCourt Night Court]] out of fear that she will become a tyrant little better than them. Trixie calls her out on her FatalFlaw of irrational fear ''At the Grand Galloping Gala'', being told that YouAreBetterThanYouThinkYouAre and that the willingness to [[IDidWhatIHadToDo Do What Has To Be Done]] would in no way make her a monster. On the other hand, Trixie's willingness to put action to word resulted in a BrokenBase, with some readers thinking that she has become as lawbreaking as the nobles.
* In ''[[FanFic/TheCrowPhoenixRising The Crow: Phoenix Rising]]'' it's {{lampshade|Hanging}}d that Harry (as the Crow's avatar) has become a monster that's just as bad if not worse than the Death Eaters he's fighting.
* Sergeant Shared Justice, or "Sherry," in Fanfic/SummerDaysAndEveningFlames. Initially only a bit overzealous when it came to Red Hooves' crime syndicate, due to [[CynicismCatalyst losing her husband to Red Hooves and watching him get out by LoopholeAbuse]], but [[spoiler:eventually turns as bad as the criminals she was hoping to weed out from Farrington. She initiates a gang war that gets dozens of her fellows police officers killed in an attempt to kill Red Hooves, shows no remorse when confronted, and ends the story with threatening to kill her former captain Iron Bulwark ([[BrokenPedestal who considered her a mother figure]]) and vowing to become the head-honcho of the Salliongrad Mafia]].
* As the title would suggest, this is a major theme in [[Fanfic/TheShroudedPath Those Who Fight Monsters]].
* Many a MarySueHunter hunter is fated to become as much as a WishFulfillment, hollow fantasy as the MarySue they were sent after.
* Harry Potter in ''[[https://www.fanfiction.net/s/6254783/1/Rise-of-the-Wizards Rise of the Wizards]]'' [[spoiler: after defeating [[BigBad Voldemort]] and coming to power he decides its time to have his revenge against the muggle world.]] [[IronicEcho "Magic is Might."]]
* At the end of ''Fanfic/WhiteDevilOfTheMoon'', it's implied that [[spoiler:Luna]] might take this path after [[spoiler:[[ScrewDestiny Nanoha decides to not resume her past duties as Princess Serenity and revive the Moon Kingdom]].]]
* Light Yagami ([[spoiler:the [[MemoryGambit innocent amnesiac]] version]]) in ''[[http://archiveofourown.org/works/637309/chapters/1154622 The Better to Kill You With, My Dear]]'' begins with high ideals and moral principals but he slowly begins JumpingOffTheSlipperySlope once [[ChronicVillainy again]] after [[spoiler: L's kidnapping]] and [[FromBadToWorse it gets worse]] when they [[spoiler: retrieve L only to find what Beyond has done to him.]] In his despair and desperation he changes his mind about making use of Watari's "[[JackBauerInterrogationTechnique enhanced interrogation techniques]]." This coupled with his growing obsession and codependence on L is [[SlowlySlippingIntoEvil slowly turning him into a monster even more twisted]] than [[EvilMeScaresMe Kira]] ever was.
* Discussed and rejected in ''Fanfic/PartingWords''. Celestia finds it ridiculous that people can believe standing up to bullies would make one as bad as them.
* The Dee-Dee Morgendorffer (Daria's school nickname) of the [[LongRunner long-running]] {{Daria}} shared-world series TheHallowedHallsOfFielding fits this trope like a glove. In a world of horrible students, an administration and parents that make looking in the other direction a honored tradition and corruption of everyone in the manner of a virus, Daria (a middle-class girl there because of her grandmother's footing the bill) decides that she's not happy with the way things are (even though the system's been working for a couple of hundred years). In the space of two years, she's caused a few disruptions so severe that they've literally changed the face of the school (and the society they service forever)...
* ''[[FanFic/MLPNextGenerationKnowFear MLP Next Generation: Know Fear!]]'': It's been noted both in-universe and out that the more Starburst uses her [[Franchise/GreenLantern Fear ring]], the more she becomes like the [[BloodKnight griffons]] she's fighting. [[spoiler: Which she comes to realize herself when [[TheEmperor Emperor Stratus]] gives her a NotSoDifferent speech when she's about to kill him in cold blood. She decides she's better than this and doesn't kill him... at which point [[TheStarscream Nox/Shadow Wing]] ''does'', in order to make Stratus a martyr and to frame Star.]]
[[/folder]]

[[folder:Films -- Animated]]
* ''WesternAnimation/BatmanMaskOfThePhantasm'': Alfred also refers to the Nietzschian quote.
-->'''Alfred:''' Vengeance blackens the soul, Bruce. I've always feared that you would become that which you fought against. You walk the edge of that abyss every night, but you haven't fallen in and I thank heaven for that.
** And this is what happened to [[spoiler:Andrea.]]
* ''WesternAnimation/BatmanUnderTheRedHood'', like ''Phantasm'', examines this Trope.
* Creator/DreamworksAnimation's ''WesternAnimation/MonstersVsAliens'' has an interesting inversion in General WR Monger, who spent 50 years as the warden for the titular Monsters, and ended up just as nice and eccentric as the monsters themselves.
* Agent Kent Mansley from ''TheIronGiant'', whose fanatical anti-communist efforts made him unable to accept the titular robot's overtures of peace, even when the general he was advising was willing to stand his soldiers down. He then [[spoiler: got a nuke launched at a small American coastal town. [[TooDumbToLive Where he happened to be.]]]]
* ''WesternAnimation/JusticeLeagueCrisisOnTwoEarths''. Batman references the Nietzsche quote when he gives a KirkSummation to his counterpart Owlman:
-->'''Batman:''' There ''is'' a difference between you and me. We both looked into the abyss, but when it looked back at us... ''you blinked''.
[[/folder]]

[[folder:Films -- Live-Action]]
* ''Film/TrainingDay'' Central premise of the film revolves around this trope.
* ''Film/MeanGirls'': a PG-13 teen comedy example:
** Cady of starts off infiltrating a GirlPosse to get revenge on the AlphaBitch, Regina, but over the course of the movie, eventually becomes a cruel, mean-spirited Libby herself.
** Janis qualifies just as much, and even caused Cady's example to start with.
* ''Film/MeanGirls'' 2 also has this. Tyler points out in one scene that Jo is acting a lot like Mandi.
* James Cameron's ''Film/TheAbyss'' is a literal and figurative interpretation of this. The movie opens with part of the Nietzsche quote[[note]]the home video version did, not the theatrical release, thanks to another movie using it that year[[/note]], and the plot revolves around civilians and [=SEAL=]s looking into an actual abyss & finding intelligent life. As they analyze the water creatures and debate over whether to destroy them with a nuke, the creatures silently watch humans and civilization on the surface above. In the longer cut version on the special edition DVD, the creatures very briefly threaten all of humanity with destruction, after having studied it for some time and realizing they might be in danger themselves. When you consider that current evolutionary theory states all carbon-based life probably originated in the oceans, this trope takes on multiple layers of meaning with regards to survival and destruction.
* ''Film/AllTheKingsMen'' in a nutshell: Willie Stark counters corrupt politicians but becomes one in the process.
* Knockout Ned from ''Film/CityOfGod'' slowly turns into this after his girlfriend was raped and his house was shot up.
* [[spoiler:Two-Face]] in ''Film/TheDarkKnight'' goes so far as to {{foreshadow|ing}} this with a line similar to Nietzsche's.
-->'''[[spoiler:Two-Face]]:''' You either die a hero, or live long enough to see yourself become the villain.
* ''Film/BatmanForever'' has one almost as good when Bruce lectured Dick about what would happen if he killed Two-Face.
-->"You make the kill. But your pain doesn't die with Harvey, it grows. So you run out into the night to find another face, and another, and another... until one terrible morning, you wake up and realize that revenge has become your whole life. And you won't know why."
* Played straight with MichaelKeaton's [[Film/{{Batman}} interpretation of]] {{Film/Batman|Returns}}, who doesn't adhere to the "No Kill" policy to the point of ''smiling'' when he killed a fat guy via dynamite.
* Kevin Bacon's character from the 2007 film ''Film/DeathSentence''. {{Lampshade|Hanging}}d by the gang leader in the ending, who, in a NotSoDifferent moment, revels at the fact that he has reduced the protagonist to such a pathetic shell of a man.
* Sheriff John Quincy Wydell in ''Film/TheDevilsRejects'' is this trope to a 'T'.
* ''Film/TheElementOfCrime'' is a particularly dark and cruel example of this trope. [[WhatCouldPossiblyGoWrong Thinking like a criminal is perhaps not such a good profiling method when the criminal is a child killer]].
* In ''Film/FailSafe'', Professor Groeteschele is arguing with Air Force General Black over the merits of launching a first-strike nuclear attack against the Soviet Union in the wake of a technical malfunction that sent a U.S. bomber to drop a bomb on Moscow, arguing that the threat posed by Communism justifies it.
-->'''Groeteschele''': How long would the Nazis have kept it up, General, if every Jew they came after had met them with a gun in his hand? But I learned from them, General Black. Oh, I learned.
-->'''Black:''' You learned too well, Professor. You learned so well that now there's no difference between you and what you want to kill.
* Tyler Durden of ''Film/FightClub''. By the end, he's shaped his group to be just as conformist as the consumerist society he's trying to overthrow, and in some cases, it's even worse.
* ''Film/{{FIST}}'': over the course of the film, the title labor union begins to act more and more like the {{Corrupt Corporate Executive}}s they were striking against in the beginning. This is driven home in the scene where they break up a wildcat strike (a strike unauthorized by the rest of the union) by force.
* By ''Film/Halloween5TheRevengeOfMichaelMyers'', Dr. Loomis is ready to use blackmail, threats, and physical force to make sure Michael Myers is put down for good. It goes so far that he uses Jamie, a 9 year old girl, as bait to lure Michael into a trap, and then beats him savagely with a plank of wood until Michael was unconscious. And yet he continues to beat him, all while screaming "DIE! DIE!" for each hit.
* The AntiVillain from ''Film/LawAbidingCitizen''. In fact, it seems like illustrating this point was his entire plan (beyond the immediate revenge on the men who raped and murdered his family). Just look at his satisfied reaction to be turned down for a plea or any sort of deal at the end. He wanted to shake the system so hard, the broken bits would be exposed, showing everyone who would look the kind of monsters that people become and allow when they start making compromises on Justice. Also, he was [[InherentlyFunnyWords nutso-bonkers]].
* Yuri Orlov in ''Film/LordOfWar'' remarks upon this in reference to the revolutions in Africa:
-->'''Yuri''': "I guess they can't own up to what they usually are: a federation of worse oppressors than the last bunch of oppressors. Often, the most barbaric atrocities occur when both combatants proclaim themselves freedom-fighters."
* Will Graham in ''Film/{{Manhunter}}'' and ''Film/RedDragon'' is a criminal profiler who lives in fear that his understanding of the mind of a killer will turn him into a sociopath.
* Creator/StevenSpielberg's ''Film/{{Munich}}'' controversially depicts "Operation Wrath of God", the covert assassination operation conducted by the Israeli government in retaliation for the murder of 11 Israeli athletes at the 1972 Summer Olympics in Munich, as being an example of this.
* The {{HBO}} movie ''[[http://www.imdb.com/title/tt0092038/combined Sword of Gideon]]'' covered the same ground.
-->'''Avner:''' If we keep taking an eye for an eye like this, pretty soon the whole world will be blind.
* In ''Film/NaturalBornKillers'', serial-killer-obsessed Jack Scagnetti, who is tracking the film's VillainProtagonist duo, eventually [[spoiler:murders a prostitute to get an idea of the thrill of killing]].
* ''Film/ThePurge'': When a gang of psychopathic killers invades your home and intends to kill you, it's a good question of how far will you go to protect yourself.
* Played with in the movie ''Film/{{Ravenous}}''. The villain gained power through cannibalism and the only way the protagonist can fight him is by partaking in cannibalism himself.
* In ''[[Film/RedDawn1984 Red Dawn]]'', Tanner, a professional military officer, is concerned about this regarding the kids:
--> '''[[ColonelBadass Tanner]]:''' "All that hate's going to burn you up, kid."
--> '''[[AxeCrazy Robert]]:''' ''(carving 'kill' notches on his AK-47 with a balisong)'' "Keeps me warm."
* ''Film/RedHill'' has this as an important part of the plot. [[spoiler: Jimmy Conway's pregnant wife is murdered and raped, and he is set on fire. He doesn't die, but spends 15 years in prison after being framed for his wife's murder before he escapes and systematically hunts down and kills the men responsible.]]
* Van Zan from ''Film/ReignOfFire'' will do anything necessary to bring down the male dragon, including press-ganging members of Quinn's homestead when not enough of them volunteer.
* Referenced by the Free French member Private Leroux -- aptly nicknamed "Frenchie" -- of the RagTagBunchOfMisfits in the 1995 remake of the movie ''Film/{{Sahara}}.'' He was sent by LaResistance to Africa to fight as a regular soldier since he had "too much hate [for the Germans] to make good Resistance."
* ''Film/{{Serenity}}'': The Operative is fully aware that in his quest to make the Alliance safe for its citizens he's fallen into this, and states that on his list of monsters, he's right there at the bottom of the list and [[NoPlaceForMeThere will kill himself when he's done.]]
--> "[[FriendlyEnemy Malcolm]], [[HeelRealization I'm a monster.]] [[IDidWhatIHadToDo What I do has to be done.]]"
* ''Film/StarTrekFirstContact'': Picard is so blinded by rage and vengeance that he becomes as cold and unsympathetic as the enemy he's fighting, to the point of alienating (heh) his officers and refusing to consider the plan that represents their best chance of success.
--> '''Lily:''' Jean-Luc, blow up the damn ship!
--> '''Picard:''' No! '''[[BigNo NOOOOOOOOOOOOO!!!]]'''
* ''Franchise/StarWars'': ''Film/TheEmpireStrikesBack'': the vision in the cave on Dagobah is a warning that Luke can become [[NotSoDifferent a monster just like Vader]] should he give in to the dark side, no matter the reasons.
-->'''Yoda:''' If you choose the quick and easier path as Vader did, you will become an agent of evil, and the galaxy will plunge into the abyss of hate and despair.
* In ''Film/ReturnOfTheJedi'', the moment when Luke strikes off Vader's mechanical hand and then stares at his own is when he realizes that he's right at the edge of that abyss, with his toes hanging over.
* Lampshaded by Obi-Wan Kenobi in ''Film/RevengeOfTheSith'': Right before his duel with Anakin, he points out that Anakin has become the very thing he sought to destroy, a Sith Lord. (For one thing, he not only slew the Tusken Raiders responsible for Shmi's death, he also "killed the women and the children.")
* ''WesternAnimation/SupermanVsTheElite'': Superman shows [[BewareTheSuperman how frightening]] he can be to people who [[BecarefulWhatYouWishFor wished he would cross this line]].
* The eponymous character of ''Film/SweeneyToddTheDemonBarberOfFleetStreet'', who starts the movie only wanting revenge upon Judge Turpin for sending him away to Australia and raping his wife, starts committing the murders that would mark him as the Demon Barber of Fleet Street after "Epiphany" and "A Little Priest", finally going off the deep end at the climax of the movie when, [[spoiler:after finally exacting ''bloody'' vengeance upon Judge Turpin, he tries to murder his own unrecognized daughter, Johanna Barker, and then, after Mrs. Lovett screams down below and unwittingly saves her, he investigates the scream and finds out that the beggar woman that he killed just prior to killing Turpin was actually Lucy, the wife that Mrs. Lovett told him [[ExactWords had died from the poison she took]] because she wanted him for herself. Sweeney, in a truly dark rage, throws Mrs. Lovett in her own oven to be burned alive. Then he settles down next to Lucy's lifeless body, where he gets killed by little Tobias Ragg, who saw the whole scene in the bake-room from the sewers after having to endure a ''serious'' [[BreakTheCutie ordeal to boot]].
* Sarah Connor in ''Film/Terminator2JudgmentDay''. Her personal mission was to stop Judgment Day from occurring and keep Skynet from forming. To accomplish this, she hid from the law, [[{{Tykebomb}} raised her only child to become a military leader,]] shacked up with gunrunners, and traded sex for stockpiles of illegal weapons, and, at her worst, almost killed an innocent man (whose life's work caused Judgment Day to happen) in front of his wife and son. That last point is particularly important, because she was trying to kill what is essentially the "father" of Skynet before he could create it. Which is ''exactly'' what Skynet sent the first terminator to do to her before she could "create" John. And the way she tries to kill said man: [[NotSoDifferent she just slowly walks after him, completely serene as she takes shots at him. Even the gun she uses (a .45 long-slide pistol with laser sight) is the exact same one that the Terminator from the first film used to try to assassinate her.]]
* ''Film/ThirteenDays''. Dean Acheson, one of Kennedy's advisors, saying that the country's been "at war with the Russkies for twenty years", advises Kennedy to deal with the Cuban Missile Crisis, essentially, by invading Cuba and hoping that Russia doesn't have the nerve to let the retaliation lead up to nuclear war. His justification was that "the only thing [the Russians] understand is force." In real life, Acheson was a tad bit more moderate, with this reasoning more in line with colleague Curtis [=LeMay=].
* This is one of the most important themes in Park Chan-Wook's ''Vengeance trilogy''.
** In ''Film/SympathyForMrVengeance'', the father sees his daughter on a coroner's slab and nearly faints. Later, after he's resolved to revenge, he watches the autopsy of another child and yawns.
** Most obvious in ''Film/{{Oldboy}}''. Both Oh Dae-su and his target Lee Woo-jin are [[GenreSavvy aware of the trope]]. Oh plainly states "Now I have become a monster." Lee utters the meaningful line "Farewell, Oh Dae-Su," and refers to him as "Mr. Monster" from then on.
** In ''Film/SympathyForLadyVengeance'', it is a crucial element of the plot. The movie doesn't end with the main character's revenge, but with her realisation of what she has become, and her vow to finally clear herself of sin.
* In ''Film/XMenFirstClass'', Magneto hates Shaw and wants to kill him, but he eventually embraces Shaw's beliefs about mutant supremacy. It's even spelled out through the villain [[ArtifactOfAttraction wearing the same helmet]] that [[IconicItem Magneto is associated with]]. At the crucial moment, he separates revenge from his ideals, which is why he's able to complement Shaw's vision while still hating the man to his core. Shaw-the-man wronged him terribly, but Shaw-the-visionary is inspirational.
* This seems to be the overall theme of ''Film/StarTrekIntoDarkness'', with the "monsters" being both [[spoiler:Khan]] and the looming threat of war with the Klingons. Both Kirk and Spock come dangerously close to becoming much like that which they were fighting. [[spoiler:Kirk's intended vengeance against Khan for killing Pike before Spock talked him out of it, and Spock's intended vengeance against Khan until Uhura talks him out of it. Admiral Marcus, in his endeavor to steel the Federation against the probability of war with the Klingons, ends up becoming a warmonger himself.]] In the epilogue, Kirk even acknowledges it during his eulogy at the funeral:
--> '''Kirk''': There will always be those who mean to do us harm. To stop them, we risk awakening the same evil within ourselves. Our first instinct is to seek revenge when those we love are taken from us...
* This is basically [[spoiler:Matias's]] character arc in ''Film/TheEliteSquad''. He originally believed in justice, but the more he learns about the corruption of the normal police and the depravity of the drug dealers, the more he dies inside. After some losses that hit close to home, he snaps. By the end of the film he's fully embraced BOPE's brutal ways.
* In ''Film/TheStarChamber'' Hardin starts to feel they're becoming this when the secret court won't stop the killing of two innocent men in order to protect themselves and because they rationalize them as likely deserving it anyway.
* Aberline spends most of ''Film/TheWolfman2010'' bent on killing Lawrence, [[spoiler:but in the end he's bitten by Lawrence and the curse is passed on to him.]]
* ''Film/{{Highwaymen}}'': When Cray saves Molly from Fargo (a serial killer who uses his car as a weapon) the first time, Fargo offers another face-to-face meeting in exchange for the girl. When Cray takes him up on the offer, Fargo laughs and notes that Cray is becoming more like him the longer he's pursuing him.
[[/folder]]

[[folder:Literature]]
* As mentioned in film section above, Willie Stark from ''Literature/AllTheKingsMen'' becomes the kind of politician he once meant to oppose.
* Bartemius Crouch, Sr. from ''Literature/HarryPotter'', the head of Magical Law enforcement during the first Wizard War, was later depicted as "having become almost as bad as those he was fighting", authorizing torture and the use of lethal force. Trials presided by him tended to be KangarooCourt and some accused Death Eaters (Sirius Black, for example) weren't even granted any before being left to rot in Azkaban.
* {{Subverted|Trope}} in ''Literature/ASeriesOfUnfortunateEvents'', in which the quotation of the above aphorism is enough to convince the heroes not to drop a villain into a literal abyss.
* Robert Neville, protagonist of Richard Matheson's famous vampire novel ''Literature/IAmLegend'', is seen this way for killing both the feral vampires and those who have enough sanity to attempt a rebuilding of society. In his defense, he didn't have a way or an incentive to tell the difference. The novel ended with [[spoiler:him being captured by the 'sane' monsters and realizing that they exist before they execute him. The title of the novel is based on his realization that he is a Legend...a legendary monster to them.]]
* The Literature/{{Eisenhorn}} novels set in the TabletopGame/{{Warhammer 40000}} universe:
** They chronicle, in first person, the struggle of Inquisitor Eisenhorn against the vile forces of [[TheCorruption Chaos]] while attempting to avoid being corrupted by them himself. As the series progresses, he shows himself more and more willing to use the devices of Chaos against itself, applying a sort of "ends justifying the means" logic to his actions.
** The danger of this happening is given as one of the reasons that the Imperium's Inquisition is so prone to slaughter everyone associated with an outbreak of Chaos, sometimes including the soldiers who helped them fight against it, since association with most enemies of the Imperium, but particularly Chaos, damns one in the eyes of the authorities. The ''Literature/GauntsGhosts'' novels also deal heavily with this trope, particularly in ''Traitor General'' and the later novels.
** This trope is, in fact, held to be inevitable by Eisenhorn and his protege, Literature/{{Ravenor}}. Ravenor explicitly stated that the [[JumpingOffTheSlipperySlope Jump Off The Slippery Slope]] is inevitable when you spend so long fighting monsters - what matters is how much good you do beforehand.
* In ''Literature/DoAndroidsDreamOfElectricSheep'', the book that inspired the film ''Film/BladeRunner'', Decker meets a fellow bounty hunter, Phil Resch, who hates Replicants a good deal more than he does. [[spoiler: He's got a good reason for that, and, considering what the other guys who went through the experience were like, he got lucky.]] Deckard often doubts that he is any more human than the [[WhatMeasureIsANonHuman replicants]] he is hunting. Which, appropriately enough, makes him one of the most human characters in the story.
* The philosophy teaching love interest in Jeffery Deaver's ''Garden of Beasts'' mused on the eponymous Nietzsche quotation with reference to the hitman main character.
* Subverted in Creator/TerryPratchett's non-''Literature/{{Discworld}}'' novel, ''{{Nation}}''. The heroine has been warning the hero about the BigBad, who, as typical in Pratchett, is far more of a monster than any creature with a face full of tentacles. In the middle of fighting said BigBad, this goes through the hero's head:
-->It was a strange, chilling thought, dancing across his head like a white thread against the - terrible red background. It went on: He can think like you. You must think like him.\\
But if I think like him, he wins, he thought back.\\
[[spoiler: And his new thought replied: Why? To think like him is not to be him! The hunter learns the ways of the hog, but he is not bacon. He learns the way of the weather, but he is not a cloud. And when the venomous beast charges at him, he remembers who is the hunter, and who is the hunted!]]
* Madame Atomos, the most famous creation of writer André Caroff, is a YellowPeril villain whose goal is to take revenge on America for the many lives lost during the bombings of Hiroshima and Nagasaki. This includes Atomos's husband and children.
* ''Missionaries'', by Lyubov and Yevgeny Lukin. Nerdy guys found a portal into the past (''as they [[AlternateUniverse thought]]'') and tried to stop an European colonization via giving the locals-to-be-colonized [[BambooTechnology somewhat more advanced]] weaponry. They succeed... but the local development was a bit faster and [[AlternateTechline not in the way]] than they imagined -- and the European exploration slower. So it ended up much the same way, only with the roles changed, "ethanol-powered turbine polymaran rocket plane carriers vs. caravels" being an obvious CurbstompBattle.
--> How it came to this? How we who hated missionaries became missionaries ourselves before we knew! Missionaries of rocket launchers...
* In ''Literature/CodexAlera'', by Jim Butcher, it turns out that Attis Aquitane, one of the villains trying to overthrow Gaius Sextus the First Lord, came to be where he is because [[spoiler: he was one of the best friends of Septimus, the assassinated Princeps. He was so disgusted with the corrupt politics of the nobility and Sextus's refusal to do away with it that when Septimus was killed, he decided the best way to end it was by using that same corruption to take over as First Lord himself.]]
* The Children of the Light of ''Literature/TheWheelOfTime'' also fit: originally, they were created to find and destroy Darkfriends, but by the time the story takes place, they have begun to persecute anyone who can use the Power.
** And Rand himself almost invokes this trope, until [[spoiler: he subverts it gloriously at the end of ''The Gathering Storm'']].
* In the ''Franchise/StarWarsExpandedUniverse'', Jacen Solo eventually comes to the conclusion that the only way to save the galaxy from an evil dictator is to attempt to become one himself. This is one of the leading reasons for his fall to the dark side and conversion to Sith.
** This is fairly common throughout the Star Wars EU - namely, characters concluding that the power of the dark side is the answer in order to defeat a greater evil. See [[spoiler:Luke Skywalker (several times, always ending with a HeelFaceTurn), Yuthura Ban, Quinlan Vos, Darth Revan (possibly), Depa Billaba...]] A quote attributed to Yoda is a paraphrase of the passage from Nietzsche: "When you look at the dark side, careful you must be… For the dark side looks back."
** This is also what led Anakin Skywalker to become Darth Vader.
* At the end of the ''TabletopGame/{{Warhammer 40000}} Literature/GreyKnights'' novel ''Hammer of Daemons'', Alaric expresses concern that the plan he concocted to bring down Drakaasi's Chaos lords and escape makes him less of a Grey Knight. He even outright compares himself to a rebellion-fomenting cultist.
* Alan Blunt, head of MI6 in the Literature/AlexRider series is willing to engage in a number of questionable practises in a bid to get reluctant teen spy Alex Rider to go on the missions Blunt wants him to undertake. He finally crosses the MoralEventHorizon in ''Scorpia Rising'' when [[spoiler:he stages a school shooting (injuring one of Alex's friends) in a bid to convince him to do one last mission.]] At the end of the novel, when he is confronted by his second-in-command Mrs Jones over his actions, he recites the Nietzsche quote.
* The Kingpriest from ''Literature/{{Dragonlance}}'' started out as a MessianicArchetype, but as he became increasingly confronted with corruption in the world, his quest to purify it became more and more [[KnightTemplar unhinged]]. In the end, he was hardly better than the people he was fighting and [[TheCaligula was completely insane to boot]]. The scary thing was, up to the very end, he was ''still'' charismatic enough to convince people that he was still the same kind and pious man who took the throne decades ago.
* This is the fate of [[spoiler: Captain Kennit]] in the ''[[Literature/RealmOfTheElderlings Liveship Trader]]'' trilogy by RobinHobb. He's an interesting case in that most of the monsterfighting took place before the trilogy starts, and is only revealed in flashbacks. So, severe overlap with FreudianExcuse in this case.
* Robert S. Pierre and Oscar Saint-Just in the ''Literature/HonorHarrington'' series, although it is very difficult to realize this. Unlike most examples, these two were ''never'' "good guys", but they were originally explicit in their devotion to ''{{Pragmatic|Villainy}}'' [[PragmaticVillainy Villainy]].
** They did not oppose the Legislaturalists because they were an evil regime, but because they were an ''incompetent'' regime that could not adequately govern the state of the Peoples Republic of Haven, which was on a fast track to complete collapse. Saint-Just himself says that he does not care who holds power, or what they use it for, as long as they use it ''well''. However, once they staged their coup and assumed complete control, they began to fall into the exact same traps and patterns of the Legislaturalists, including the promotion of personnel based on their political connections instead of skill, and the forgiveness of their errors because of those same connections. That specific action was one of the [[TheLastStraw final straws]] that instigated their coup, but their planned reforms and actions are slowly pushed further and further into the background as they become more and more preoccupied with simply maintaining control.
** Saint-Just crosses the line for his actions as head of [=StateSec=] and for [[spoiler:killing millions of people with an atomic weapon to put down the [=McQueen=] coup]]. But with Pierre, we get enough of a portrait of his origins, original idealistic intentions, and failure to fix a system that just can't be fixed, that we [[AlasPoorVillain feel somewhat bad when he dies]]. The Pritchart administration even admits that Pierre's policies as Chairman improved the Republic's economy and education in the long run, which redeems him a bit further.
* In Philip Pullman's ''Literature/HisDarkMaterials'', two of the major nemesises--Lord Asriel and Marisa Coulter--embody Sydney Harris's maxim over the course of the trilogy. [[spoiler:Also, Nietzsche's reference to The Abyss will become particularly relevant.]]
* It's implied (or at least believed by non-witchers) that the three main causes of death for [[TheWitcher Witchers]] are monsters, angry/scared peasants (as a monster), and other Witchers.
** The Scoia'tael, brigades of nonhumans (elves and dwarves) that resist what they see as man's occupation of traditionally Elder Race lands can also count as this. Though they portray themselves as freedom fighters, they are extremely brutal, constantly attack civilians, and get non-Scoia'tael nonhumans in trouble due to suspicion that any of them could be a sympathizer. Some of them even attack non-Scoia'tael elves and dwarves.
* In Jim Butcher's ''Literature/TheDresdenFiles'', the book ''Literature/FoolMoon'' has [[spoiler:an FBI team who turn into literal monsters in order to gain an edge on the Chicago mafia, and, as a result, lose their minds]].
** The ''Dresden Files'' novels also have [[spoiler:Hellfire]]. When Harry uses this to enhance his spells, it also enhances and fuels his anger at the same time. At its climax, Harry destroys something major out of anger and is chilled because, in the Dresden 'verse, you cannot cast a spell unless you truly deeply believe it is right. It is then he realizes the influence [[spoiler:Hellfire]] has upon him.
** Now that Harry's [[spoiler: the Winter Knight]] and has gained a lot more power, various creatures having begun referring to him as a "monster," and Harry himself feels that he has started to change into something that he once would have fought. Time will tell if he manages to retain his inherent goodness.
** Harry spends some time thinking about this trope in ''Ghost Story'', considering all the lines he's crossed along the way, most recently [[spoiler: becoming the Winter Knight, deliberately murdering Susan in order to exterminate the Red Court and talking Molly (who's been in love with him for years,) into helping him arrange for his own assassination]]. His eventual conclusion is that [[spoiler: making hasty mistakes to save his daughter at the end of a life he devoted to helping people does not make him a bad person]].
* The main character arc of Literature/AnitaBlake over the course of her eponymous series was her either becoming a sociopathic serial killer who just happened to have a socially acceptable victim profile or [[AlternateCharacterInterpretation realising that she was this all along]]. At the end of the [[FanonDiscontinuity last book of the series]], Otto, another serial killer, acknowledges her as a colleague, and she can't really deny it.
* Crixus in ''Emperor: The Death of Kings'' comes close - he intends to take his revenge on the wealthy Romans by taking the slave army to live in their grand houses. Spartacus points out that since those big houses and farms need slaves to maintain them, living in one would make Crixus just as bad as the senators he hates so much.
* ''Literature/TheHungerGames'':
** [[spoiler:District Thirteen]] does a bit of this with their rules and war tactics. Katniss also calls [[spoiler:Gale]] out on his tactics and standards that are very similar to that of their enemies.
** [[spoiler: Katniss and some of the other Hunger Games victors appear to be this]] when they decide to [[spoiler:hold another Hunger Games with Capitol children in order to get revenge for the district's suffering. It is a part of the {{batman gambit}} she uses to fool President Coin into thinking she's on her side.]]
* In Literature/TheSagaOfDarrenShan, this is how Evanna [[InvokedTrope predicts]] [[spoiler: [[ApocalypseMaiden Darren]]'s]] StartOfDarkness as the Lord of the Shadows, [[spoiler: should he defeat [[TheRival Steve]]. Ultimately, he [[DefiedTrope defies it]] by [[ScrewDestiny dying]] [[TakingYouWithMe along with Steve]].]]
* A major theme of ''Literature/{{Animorphs}}''.
** The protagonists often worry about how to defeat their enemies without becoming just like them.
** Alloran, who committed genocide to prevent the Hork-Bajir from falling into Yeerk hands, is said to have been an idealist before the Yeerks's betrayal and slaughter of his comrades drove him over the DespairEventHorizon.
** [[BloodKnight Rachel]] became this. Before the final mission, she accepts the fact that she's become something of a nutcase and thinks that Jake's made the right decision in using her appropriately. To illustrate: she's been harboring worries about whether she could even ''function'' without the war anymore, and it's all too clear she is not exactly a paragon of mental health, even among the Animorphs, who have all spent the last ''three years'' [[HoldTheLine holding the line]], fighting a guerrilla war against a powerful alien empire. It's even possible that she's been insane at least since the events of the last book she narrates, ''The Return'', after being ''horrendously'' {{Mind Rape}}d by [[spoiler:[[AxCrazy David]], who was working as a pawn for [[OmnicidalManiac Crayak]],]] to get revenge.
** Depending on your [[AlternateCharacterInterpretation interpretation]], [[spoiler:Jake]]'s actions in the final arc, particularly the penultimate book of the series, may place him here as well.
* ''Literature/TwentyThousandLeaguesUnderTheSea'': Captain Nemo MajoredInWesternHypocrisy and wanted revenge against TheEmpire. He creates an NGOSuperpower with an OddlySmallOrganization with her own ConLang. [[EgoPolis He claims a continent in his name,]] creates the Nautilus to conquer TheFinalFrontier (the sea) and to use it as a WeaponOfMassDestruction, [[FromMyOwnPersonalGarden insists on only using sea-related products]], and the prisoners he considers valuable are placed in a GildedCage, but those who not are [[KickTheDog mercilessly destroyed]]. In trying to destroy TheEmpire, he ends up [[StartMyOwn creating a society]] [[MilkmanConspiracy very much like it]].
* The Literature/DoctorWhoNewAdventures novels, which featured a Doctor who, in the name of defending the universe from evil, would not only ShootTheDog but subvert history over a hundred years to make sure the dog and the gun were in the right place, and then [[EarthShatteringKaboom blow up the planet]] just to be on the safe side, often contemplated the Nietzsche quote.
* Literature/HerculePoirot knew he possessed both the ability and the ego to become the very kind of killer he was always working to put behind bars. [[spoiler: It's why he deliberately hastens his own death in ''Literature/{{Curtain}}'', to ensure he never gives into this kind of temptation.]]
* Sir Alistair Ravenscroft from ''Literature/ElephantsCanRemember'' also averts this trope by deciding he must pay for his [[spoiler:[[NecessaryEvil killing of Dolly]] with his life.]]
* Literature/SisterhoodSeries by Creator/FernMichaels: The Vigilantes are a group of women and one man who have been wronged in a number of ways. They decided to get {{Revenge}} on those who wronged them. That's right, they're not really pretending to be doing this for justice. They definitely have this happen to them in ''Vendetta'' by skinning John Chai alive. He was a creep and a SmugSnake, but the DisproportionateRetribution inflicted on him caused the group to sink to his level.
* In the Literature/NewJediOrder arc, the Yuuzhan Vong are so horrible and implacable that those willing to recognize that there is nothing to be gained and much to be lost by appeasement face the temptation to [[JumpingOffTheSlipperySlope Jump off the Slippery Slope]]. It turns out in the final book that this trope was the Yuuzhan Vong's own StartOfDarkness.
* In ''Literature/OutOfTheDark'' Mircea discusses the trope and expresses gratitude that he did not fall to it.
* Ha Jin's short story "Saboteur" features a newlywed, hepatitis-afflicted professor during the Cultural Revolution who is arrested by cops for "disrupting the peace" (in reality, said cops disrupted the peace by throwing hot tea at the professor and his wife's feet and arrested him when he rightly protested). The professor eventually leaves jail but then leaves mostly uneaten bowls of food at various locations. Later on, many people contract hepatitis with a few dying, including children.
* A more mundane example occurs in ''Telling Liddy'' by Anne Fine, when Bridie, the heroine, spends much of the book raging against her sisters after Liddy, the baby of the family, falls out with her, and Heather and Stella, the other two sisters, go along with it, all because of a ''rumour'' that Liddy's husband George is a paedophile. She resents Heather for her selfishness, Stella for enabling Liddy's behaviour, and Liddy for cutting her off even though, as a social worker, she was trying to do the right thing by warning her about the rumour. [[spoiler:We later find out that part of the reason why Liddy is angry with Bridie is because she had an affair with Bridie's husband Dennis, and never told her about it, and subsequently feels it's unfair that Bridie was willing to break up her relationship when she had kept silent for Bridie's sake.]] By the end of the book, Bridie realises she has become the very thing she hated:
--> Within a matter of weeks, she'd changed from being the woman he had always loved into someone as sly and calculating as Stella, as self-regarding as Liddy, as unfeeling as Heather. He had been watching her...and he had understood that she had sold her soul.
* In Nick Perumov's ''Sword Guardian'' series, [[spoiler: Silvia's father]] activated the full power of the Man Sword while trying to defeat evil creatures controlling the southern continent. This turned him into Death Rain Master, an even more dangerous monster. Later on, [[spoiler: Silvia]] nearly falls into this trap herself, but manages to retain her humanity due to outside help.
* The Egyptian freedom fighters in ''[[Literature/ThePowerOfFive Oblivion]]'', to Richard's disgust.
* Lucian Clemant, Gerry Habbentz, Colonel Zak Larraine, and Prebendant Balthazar Delastro in ''Literature/TheLambAmongStars''. The interstellar Assembly is the Millennial Kingdom prophesied in Revelation, but when evil re-entered (also as prophesied) those four, among others, quickly sank to the point where they would do anything to defeat Lord-Emperor Nezhuala and his Dominion, [[spoiler:even to the point where they meant to slaughter everyone in the Sarata system, the seat of the Dominion]]. [[spoiler:Zak has a HeelRealization (though his final fate is [[WhatHappenedToTheMouse unknown]]), Gerry is shot attacking Merral after they are exposed, Clemant commits suicide while being taken to the Moon, and Delsastro turns traitor and is DraggedOffToHell even as good triumphs.]]
* In ''[[{{Monstrumologist}} The Monstrumologist]]'' series by Rick Yancey, the quote is recited (at least once in the original German, no less) by multiple characters. It's a constant worry and very visible struggle throughout the series, appropriately enough considering that the main characters' profession is quite literally fighting (and studying) monsters, and at least one character has indeed become this: Dr. Warthrop's "friend", Dr. John Kearns, who freely admits that's not his real name and is insinuated to be ''Jack the Ripper''. Most of the other characters are still in the struggling/debatable zone, but everyone who's gone up against one of the monsters has shown some behavior reminiscent of this trope to some extent or another.
%%* ''MichaelConnelly'': [[DiscussedTrope Discussed]] in ''The Poet''.
%%** Discussed again in ''Lost Light''.
** Honey Chandler in ''The Concrete Blonde'' accuses Harry Bosch of this and quotes FrederichNietzsche.
[[/folder]]

[[folder:Live Action TV]]
* Most of the Argent family from ''Series/TeenWolf''. Their role seem to be keeping supernatural creatures in line, but can be just as cruel as the werewolves. Chris Argent is more of a KnightTemplar, but has no qualms about threatening sixteen-year-olds. Victoria is fine with torturing ordinary humans that do not even know werewolves exist just to create job vacancies for Hunters. Kate and Gerald were each an outright psychopathic ManipulativeBastard.
* Eben from ''Series/TheSecretCircle'' seems to have become this from fighting John Blackwell.
* ''Series/TheXFiles'', episode "Grotesque":
** It featured [[TheProfiler an FBI agent who became so obsessed with "getting into the head" of a serial killer that he was chasing]] that he soon turned evil and began murdering people in a manner similar to that used by the serial killer. Well either that, [[MaybeMagicMaybeMundane or he was possessed by the same demon that made the other man a serial killer]]. This was Mulder's former boss, and Mulder wound up having to stop him. He ended up being a BrokenPedestal for Mulder and other agents.
** When Mulder was working on this case, he was sinking into the darkness as well, because he also needed to get into the killer's head. Both Scully (his partner) and Skinner (his current boss) were extremely worried. Ultimately, Mulder subverts the trope - he was messed up, but he got better once the case was resolved.
* ''Series/BuffyTheVampireSlayer'':
** Dark Willow started out wanting revenge against Warren for what her did to her girlfriend, Tara, then moved onto his friends because they were nerds too, and soon escalated into CardCarryingVillain territory, until she eventually [[OmnicidalManiac tried to destroy the world]]. Willow was brought back from the abyss by [[ThePowerOfLove Xander]].
** The whole concept of a "Slayer" is based on this. A Slayer is supposed to fight demons, but her powers are demonic in nature. She is not expected to be nice to those whom she runs into in life and her life is short, nasty, and brutish. Several episodes (the one involving the First Slayer and an alternate universe version of Buffy, among others) deal with this.
** Faith's character arc embodies this, presenting her as a [[EvilCounterpart dark mirror]] to Buffy. Faith is shown to not only slay demons, but to enjoy it 'a little too much' and she is very brutal about it. This was partly because her Watcher was murdered by a demon, but also because she resented anyone having power over her.
** The Season 3 episode "Gingerbread" begins with Buffy's mom finding two young children after what looks like a magical rite. She responds by organizing the other parents in Sunnydale into an organization to go after witches (and Slayers.) The episode ends with them all trying to burn ''their own children'' at the stake.
** This literally happened to Forrest.
* ''Series/{{Angel}}'':
** Holtz is so obsessed with obtaining "justice" against Angelus that he followed him into the future, disregarded all the myriad evidence of Angel's reformation, and did all he can to make Angel suffer psychologically. Although, at the end, he seems to make a comeback when he mentions that love has overcome hate. This turns out to be a ruse; he even uses his own death as further fuel to get Connor to take his revenge for him.
** Angel himself goes pretty far into this territory in season 2, and he seems to do it deliberately, re-shaping himself into someone willing to use evil methods to wipe out evil.
* ''Series/DoctorWho'':
** The TechnicalPacifist Doctor has killed ''very many'' Cybermen and Daleks. He has annihilated various monsters of the week and entire fleets of enemy spacecraft, as well as, presumably, his own people. The Doctor seems to swing back and forth on the SlidingScaleOfIdealismVersusCynicism quite frequently. In one case, he was attacked by creatures who wanted to steal his immortality. [[AndIMustScream They got their immortality all right]]. Getting the Doctor personally angry is, in his own words, "not a good place to stand."
** As Donna says in [[Recap/DoctorWho2006CSTheRunawayBride "The Runaway Bride"]], "I think sometimes you need somebody to stop you." -- since he became the LastOfHisKind, the influence of a [[MoralityPet companion]] ideally should serve to keep him from [[MoralEventHorizon becoming too monstrous]].
** In the Tenth Doctor's final TV appearance, [[Recap/DoctorWhoS30E17E18TheEndOfTime "The End of Time"]], they go into detail about what was going on at the end of the time war. [[spoiler:After both sides had done too much messing around with [[NegativeSpaceWedgie space wedgies]] and [[TemporalParadox paradoxes]], the [[TimeyWimeyBall fabric of time]] was irreparably damaged (though in a localized area). Countless Daleks and Time Lords alike were being slaughtered over and over again in endless time loops and Gallifrey itself had basically turned into hell. When the Doctor ended the war, he sealed off the area the war encompassed in a time bubble, preventing it from extending further throughout the universe.]] If the Doctor hadn't ended it the way he did, [[spoiler:the Time Lord leadership would have destroyed all of reality because they (and they alone) would be able to survive outside of time as [[AscendToAHigherPlaneOfExistence beings of pure energy and information]]]].
** And then there's the Tenth Doctor in [[Recap/DoctorWhoS30E16TheWatersOfMars "The Waters of Mars"]]:
-->[[spoiler:'''The Doctor:''' Do you know who that leaves? ME!! It's taken me all these years to realize it, [[AGodAmI but all those laws of time are mine.]] And they will obey ME!]]
** In the episode [[Recap/DoctorWhoS31E12ThePandoricaOpens "The Pandorica Opens"]], the [[spoiler:the thing imprisoned in the Pandorica, the nameless, fearless, bloodthirsty monster so terrifying and destructive that it has become renowned as a dark fairytale, turns out to be the Doctor himself.]] To call the two-word reveal a WhamLine for the Doctor is an understatement. He seems more shaken at the idea of ''being'' [[spoiler:the thing in the Pandorica]] than at the fact that he's getting [[spoiler:locked inside.]]
** In [[Recap/DoctorWhoS32E7AGoodManGoesToWar "A Good Man Goes To War"]], he's told even more explicitly that he's turning into this, not just that people see him as TheDreaded. He even admits it himself:
--->'''The Doctor:''' Good men don't need rules. Today is not the day to find out why I have so many of them.
** Well illustrated in the Ninth Doctor episode [[Recap/DoctorWhoS27E6Dalek "Dalek"]]:
--->'''The Doctor:''' The Daleks have failed! Why don't you finish the job, and make the Daleks extinct? Rid the universe of your filth! Why don't you just die?\\
'''Dalek:''' [[foldercontrol]]

[[folder: You would make a good Dalek. ]]

** The ''less'' dark Eleventh Doctor has had to face this, and we got some good insight into his more anti-hero-y moments. Cyborg dude persecutes town to get at a guy. Cyborg proves to have a point when it comes to that guy. The Doctor is ready to hand him right over, and Amy is the one to say WhatTheHellHero. It turns out that it's less that the Doctor thinks he ''owns'' time and space because he's the last Time Lord; he feels he's the only one to protect it, and that he may as well have pulled the trigger on everyone ever hurt by those he ''didn't'' take down, like the Daleks, Master, etc. He walks a very fine line between getting the job done [[IDidWhatIHadToDo even if it means getting dirty]], and becoming a KnightTemplar. It really puts the times the Tenth Doctor made you ask "isn't he supposed to be the good guy?" in new light.
** The backstory to [[Recap/DoctorWhoS18E5WarriorsGate "Warriors Gate"]] is that when the Tharils' slaves overthrew their masters, they crushed under their own heels.
** The Rutans from [[Recap/DoctorWhoS15E1HorrorOfFangRock "Horror of Fang Rock"]] may be this in their long war with the Sontarans.
** Season 18 was precided over by a script editor with the pet idea that the Doctor was 'a [[HorrifyingHero monster that]] [[CreepyGood fights monsters]]', so every story in it indulges in this theme - some in greater detail than others.
* Happens in the episode of the ''Series/TwilightZone'' "The Mirror". In this episode, a rebel overthrows a dictator in a banana republic. However, the dethroned dictator says the rebel will learn the consequences of ruling by force (i.e. killing people to maintain power). The new ruler becomes more and more paranoid, using more and more vicious measures to maintain his rule, proving he indeed became just like the dictator he deposed.
* ''Everyone'' in ''Series/{{Supernatural}}'' has this problem ''all'' the time. It's not just the contact-with-evil, that is, the 'Monsters' part; it's also the 'Hunts' part, the ''violence'' inherent in the lifestyle. Most (if not all) hunters are this, being pushed into hunting after having a loved one murdered by one of the monsters, which leads many to be obsessed with revenge.
** Gordon Walker is the purest example, becoming worse than the monsters he hunts taking them out. For a series that can succumb to the temptation of explicitly spelling out character psychology as frequently as ''Series/{{Supernatural}}'' (how many times has someone told Dean that he lacks self-esteem, is afraid of being alone, is dead inside, yadda yadda yadda), Gordon was thankfully handled with restraint. In his three individual episodes, he comes off as just a sadistic bastard, but put them together and the story is all there: his family blamed him for letting his sister disappear (they wouldn't believe that she had been vamped), and he [[StakingTheLovedOne hunted her down and killed her]], refusing to admit that it was out of anger instead of necessity. But inside, he is so guilt-ridden that he is desperate for everyone to see the world in terms of black-and-white (which would justify his actions), with Gordon on the side of the good guys (thus his creepy obsession with getting Dean's approval).
** All the Winchesters have been like this (mixed in with that good old DeathSeeker attitude) at some point. John was this way about everything related to Mary's death.
** Dean was like this this after John died and he had that big-secret-that-totally-wasn't weighing on his shoulders, and has had such moments of ruthlessness every time his family leaves him or lets him down or he's really freaking out about his brother. Such as when he encounters Gordon in season two after his father dies; when he so loses faith in his brother that he agrees to the angels' plan in season five even though it will destroy most of the world; and in season seven when he [[spoiler:kills Amy Pond ([[Series/DoctorWho not that one]]) because he can't trust a monster not to kill again, complete with a [[Film/KillBill Beatrix Kiddo]] moment with the woman's son afterward.]]
** Sam was this after Dean died in ''Mystery Spot'' and the season three finale. While he thinks killing Lilith is the only way to prevent the Apocalypse and feeding [[PsychoSerum demon-blood]]-fueled powers also lets him save the hosts when exorcising demons, his obsession with gaining the power to kill Lilith [[spoiler:leads him to break the final seal, releasing Lucifer from Hell]].
** Future Dean in "The End" (5x04). After losing his brother and failing to stop the apocalypse, he becomes heartless and unsympathetic, willing to sacrifice all of his loyal friends for a chance to kill Lucifer.
** In seasons 6 and 7, [[spoiler:re-angelified Castiel]] has taken a particularly nasty route to this, starting with a DealWithTheDevil, moving on to murder and betrayal, and then JumpingOffTheSlipperySlope with murder and MindRape of friends even ''before'' diving into WithGreatPowerComesGreatInsanity.
* Eric van Helsing from ''YoungDracula'' is a comedic version of this.
* ''Series/{{Dexter}}'s'':
** The title character, a VillainProtagonist, inverts this, as his fight makes him ''less'' of a monster than he would be otherwise. He's more a combination of EvilVersusEvil and EvenEvilHasStandards.
** [[spoiler: ADA Miguel Prado]], however, falls into this after his brother is killed, leading him to find out Dexter's secret and learn his methods, leading him to [[spoiler: [[MoralEventHorizon kill defense attorney Ellen Wolf]],]] and [[spoiler: attempting to murder [=LaGuerta=], before Dexter kills him off.]]
* A mayor plot point in both seasons of Argentinian HBO crime series ''Epitafios'', appearing in season 1 [[spoiler: with Renzo, who murders Costas in cold-blood after his murder spree (including Laura)]] and in season 2 it comes back with a vengeance with [[spoiler: both Marina and Renzo, the former shooting her brother's murderer and the later [[MoralEventHorizon burying the main villain of the season... ''alive'']]]].
* ''Series/{{Heroes}}'':
** HRG. While much of what he does is for Claire, working to capture the monsters in Level 5 shaped him into the unscrupulous operative he is today
** Peter is also headed down this path in Season 3 of ''Heroes'', when [[spoiler: taking Sylar's power in order to save the world caused him to also gain Sylar's hunger]].
* One season 9 episode of ''Series/LawAndOrderSpecialVictimsUnit'' had a serial killer found dead in the same manner as his victims. [[spoiler:Turned out, it was the lead investigator who killed him, because her mentor committed suicide from the stress of trying to catch him. However, it meant that she inadvertently killed his last victim, who had been abducted but not killed yet. When Olivia takes her back to her apartment to get the gun, she tearfully quotes the page title word for word before blowing her brains out.]]
%%* ''[[Series/LawAndOrderSpecialVictimsUnit Law and Order: SVU]]'''s Chester Lake.
* This was the origin of the title character in ''XenaWarriorPrincess''. Xena first raised an army to protect her village from a warlord, but her brother was killed in the process. She proceeded to actively seek out possible enemies of Amphipolis and destroy them; it was not until her first encounter with Caesar that she abandoned this as an excuse.
* ''CSINewYork'' had a soldier-wannabe who went off his meds and [[TheSchizophreniaConspiracy became paranoid]] that America "wasn't ready" for a terrorist attack. So what does he do? [[KnightTemplar He plants bombs and blows people up]], while playing CriminalMindGames with Mac and the cops.
* ''Series/TwentyFour'':
** In season seven, [[spoiler: Tony]] displays this trope. To the point where he actively kills innocent people, and the FBI agents trying to find him, all so he can have revenge.
** [[spoiler: Jack Bauer]] takes this trope UpToEleven in the second half of the eighth season, when he gets his hands on a murderer. [[spoiler: Jack eventually backs down when he realizes what the consequences (to innocent people) will be if carries out his revenge]].
* The re-imagined ''{{Series/Battlestar Galactica|Reimagined}}'' saw the Resistance on New Caprica using suicide bombers against the Cylon occupation force. Colonel Tigh gives us this quote. He's being partly sarcastic, though.
-->"Which side are we on? We're on the side of the demons, Chief. We're evil men in the gardens of paradise, sent by the forces of death to spread devastation and destruction wherever we go. I'm surprised you didn't know that."
* A patient, Curtis Ames, from ''Series/{{ER}}'', was a good man who crumbled under the loss of his right arm, the divorce of his wife, his children calling another man "dad", and losing his job. He sought to [[RevengeByProxy get even with Kovac]], who had treated him.
* ''Series/DarkShadows'' has Reverend Trask, a self-styled witch hunter who had undoubtedly killed many innocent women. As a ghost, he's finally talked into a HeelFaceTurn thanks to the opportunity to finally destroy a real witch.
* ''Series/{{Lost}}'': considering who the series's ultimate BigBad is, this can be inferred as the reason for much of the Others's villainous behavior.
* ''Series/{{NCIS}}'':
** Eli David, Ziva's father, is pretty obviously this. He crossed the MoralEventHorizon, and it is obvious that he does so because of his determination to protect his people against vicious enemies.
** Jenny Shepherd is this about Rene Benoit.
* In ''Series/CriminalMinds'', the Nietzsche quote is used twice, once in the first episode, and once in the hundredth episode. It's referenced in the season four finale during the finale voiceover ("How many more times will [my team] be able to look into the abyss"). However, the BAU doesn't really fit this trope, and, in the hundredth episode, [[spoiler: it's pretty clear that Hotch did the right thing]]. However, Gideon's departure from the team is due to his fear and realization that he's been staring into the abyss for too long and can no longer see humanity past it. He leaves to wander the world for a while and restore his faith in humanity.
* ''Series/TheMentalist'':
** Patrick Jane is so much this when it comes to Red John. The show even goes so far as to hang a lampshade on it in the season 3 episode "Red Moon:"
--->'''Jane:''' I have spent enough time with that creep. Staring into the abyss--you know, it's not healthy.
** The only real difference between the way the two use people is that Red John's manipulations end in murder, whereas Jane's tend to end in arrests...but also frequently destroyed relationships, families, and psyches.
* In ''Series/{{Life On Mars|2006}}'', [[spoiler:Harry Woolf]] spends much of his career as a copper watching his nemesis become rich through illegal means while he only gets a comparatively paltry wage. To make up for this, [[spoiler:he has banks robbed and blames the crimes on his enemies, has one of the underlings of his nemesis murdered, and betrays his protégé, Gene Hunt.]]
* This happened to Jim Lahey in ''TrailerParkBoys''. He was driven to great depths of depravity in his effort to save his home from the villainous machinations of Ricky and Julian. Truly, those two criminals were the shit-abyss Lahey looked into and never quite got out of.
* ''Series/VeronicaMars'' implies that Keith and Veronica's career choices are starting to take their toll on the characters's well being and sense of morality.
* In ''Series/{{Community}}'', this happens to Abed when he is taking down a number of [[AlphaBitch Alpha Bitches]] who humiliate other women. He ends up indiscriminately pointing out the flaws of everyone.
* Antonia of ''Series/TrueBlood'' was a witch who was raped and murdered by vampires in the middle ages. When she comes back as a spirit, the next logical step is to attempt genocide against the entire vampire race, attacking and imprisoning everyone that stands in her way.
* ''Series/{{Merlin}}'':
** King Uther. He lashes out at the death of Ygraine due to the magic used to conceive Arthur, and launches into the Great Purge, killing everyone in Camelot even suspected of using magic, and forever banning magic in the kingdom. Except he invented most of the 'monsters' in his grief.
** Merlin himself. During the course of the series, he has constantly lied to hide his magic, committed countless murders and on one occasion betrayal, and can be just as ruthless as his arch-nemesis Morgana. If not for his loyalty to Arthur, he could go to the very deep end.
** Likewise, Morgana started off as a heroine in series 1-2, someone who would defy the king to help, for example, Merlin's village fight off the bandits attacking it, or demand of Arthur that he save Gwen, a mere servant in Uther's eyes. As time goes on, and [[spoiler:she develops magical powers, and Uther kills others who she knows who he suspected of sorcery (notably Gwen's father and the druids who take her in),]] her attitude to Uther becomes more and more poisoned, until [[spoiler:she attempts to kill him twice. The first time, in a subversion to the trope, she performs a HeelFaceTurn when she sees that he is truly sorry for what he has done. On the second occasion, after Uther has continued as he was, she becomes fully committed to killing him]], and by the end of her training with Morgause, she is willing to manipulate and kill anyone who stands in the way of her destroying Uther, and everyone related to him.
* ''Series/PersonOfInterest:''
** Reese is a very self-aware version.
** His mentor Cara Stanton was even more aware of it, and even tells him such when he joins the CIA: "We don't walk in darkness, we ''are'' the darkness."
** FBI Special Agent Donnelly is an interesting example. Being a SympatheticInspectorAntagonist, he seems to be a generally capable, upstanding lawman who is chasing what he is sure to be a well-funded terrorist group (In reality, Team Machine). However, over the course of the series his methods become more and more extreme, to the point of labeling four men who, as far as he knows, could just be bankers as terrorists and holding them against their rights, and even going so far as to almost allow a possibly innocent man to be beaten to death by prison inmates (hoping he would reveal his combat training). Eventually, he stops trusting anyone and loses the respect of Carter. [[spoiler:Donnelly finally gets his man due to his Paranoia, but because he assumed the situation was less complicated than it was, he ended up dead for his trouble.]]
* ''Series/SpaceAboveAndBeyond'' gives us Col. Ray Butts, who apparently was born mean and became meaner from being a Marine lifer--he's racist against [[ArtificialHuman InVitros]], picks pointless fights with the Wildcards, antagonizes [=McQueen=] by taking the squad away from him for a mission he won't explain to anyone, and changes mission parameters mid-mission, again without any sort of explanation. It gets to the point where the squad briefly wonders if he might have killed his previous squad members when they fight a dead marine's body on the planet. [[spoiler: the squad was actually killed by chigs when they wanted to wait for reinforcements--causing Butts to leave them in disgust to do the mission on his own]]
* In ''[[Series/SpartacusBloodAndSand Spartacus War Of The Damned]]'': A number of the rebels are showing signs of this, as seen when they slaughter innocent civillians including children. Spartacus wants them to be better than the Romans, but is unable to keep them in line. Gannicus is aware of what they are becoming, but seems to have resigned himself to the inevitability of it.
* ''Series/{{Arrow}}'' gives us [[ComicBook/{{Huntress}} Helena Bertinelli]], who is initially shown as a WoobieDestroyerOfWorlds type because her father killed her fiancee. Naturally, she wants revenge on her father. However, her plot involves [[MoralEventHorizon killing and maiming multiple innocent people.]] LikeFatherLikeSon at its worst.
* The lead character of ''Series/{{Hannibal}}'' -- who is not the eponymous Lecter but rather FBI [[TheProfiler profiler]] Will Graham -- is CursedWithAwesome[[AwesomenessByAnalysis ness By Analysis]]. He inspects the crime scenes of serial killers and reconstructs means, motive and pathology from them - almost literally reliving the crime as it was committed. Needless to say, what he finds in the minds of those killers is pure NightmareFuel, and basically every character in the cast cautions Graham's FBI superior and Will himself about the possibility of this trope. And then we add the fact that the eponymous Lecter is TheCorrupter, who has every reason to push Graham into that abyss...
* ''Series/{{Highlander}}'' The Dark Quickening is this. It's what happens when a good immortal takes in to much evil from the others they defeat and corrupts them.
* ''Series/BabylonFive'' has this in spades:
** The first and most obvious example is the Narn. They used to be a peaceful and technologically primitive race before the Centauri conquered their planet and enslaved them. After decades of fight, the Narn managed to force the Centauri out... And promptly started using the technology they stole from the Centauri to conquer their neighbours while they prepare their revenge against the Centauri.
** [[{{Irony}} Ironically]], the Centauri themselves (whose RPG rulebook even starts with Nietzsche's trope-naming quote). Before first contact with the Xon, th other sentient race of their own homeworld, they were peaceful artists who had even rejected the very concept of war. Then a naval expedition reached the Xon lands, causing the Xon to find out about them and attack the Centauri, killing and enslaving many of them. By the end of the war, that also included a brief alien invasion from the Shroggen, no Xon was alive, and the Centauri were a fledgling empire ruled by a DeadlyDecadentCourt and bent on expansion to get even with the Shroggen and protect other races. With time, they forgot their motivation.
** The Minbari in general and their Warrior Caste in particular. After the last war against [[AbusivePrecursors the Shadows]] they spent a thousand years to prepare for the next, and just as it was coming [[PoorCommunicationKills a screwed-up first contact with Earth caused the death of their political and religious leader]], prompting them to start a genocidal war in spite of the humans trying to surrender multiple times. They stop and surrender right after destroying the last of Earth's military and a few minutes before actually stopping the genocide, thanks to [[spoiler: finding out evidence that Minbari souls are reincarnating in humans]], but, partly because the motivation was kept from the public, it takes a while for the Warrior Caste to stop behaving like everyone is beneath them (the first time we see a Minbari warship in the series, it repeats the same mistake that caused the war. Thankfully Delenn was there to explain that custom).
** The humans themselves. After the devastation of the [[HopelessWar Earth-Minbari War]], in which their allies abandoned them out of fear and the only help they received was weapons sold to them by the Narn, many humans, especially in the government, felt they had to do ''anything'' to prevent this from happening again, including [[spoiler: killing the president of Earth Alliance in a fake accident and [[DealWithTheDevil allying with the Shadows]]]].
* In the ''Series/MastersOfHorror'' episode "Incident On and Off a Mountain Road", Ellen eerily takes on many of the villain Moonface's mannerisms at the end. She gives [[spoiler:her dead husband]] the same treatment Moonface gave to his victims and kills [[spoiler:Moonface's insane captive Buddy]] to tie up all loose ends.
* In ''Series/{{Castle}}'', it's revealed that Detective Kate Beckett's mother was murdered as the result of a lengthy chain of events that resulted from a trio of cops who, cynical about the justice system's ability to effectively deal with the mob, eventually rogue in order to bring them down. In her efforts to expose the people behind her mother's death, it gradually becomes clear that Beckett is beginning to take on several similarities to these cops, including going rogue at times. [[spoiler: It's ultimately subverted; she ends up having an epiphany in which she realises she's in love with Castle, is throwing her life away on revenge and decides to step back and focus on building a life with him rather than spiral into self-destructive obsession.]]
* In the fifth season of ''Series/TheShield'', Jon Kavanaugh starts out as a well-meaning (if self-righteous) Internal Affairs officer investigating the corrupt Strike Team, especially [[VillainProtagonist Vic Mackey]]. But as the season progresses, Kavanaugh's quest to take Vic down becomes increasingly personal, desperate, and obsessive. In Season 6, Kavanaugh finally becomes a dirty cop himself, planting evidence and coercing false testimony against Vic, which leads to his own downfall. As Kavanaugh puts it in his final episode, "I framed a guilty man."
[[/folder]]

[[folder:Music]]
* Music/BobDylan's "My Back Pages":
-->''In a soldier's stance, I aimed my hand\\
At the mongrel dogs who teach\\
Fearing not I'd become my enemy\\
In the instant that I preach''
** While Dylan presumably was writing about the {{Author Tract}}s of his earlier "protest music" period, these lines would also prove oddly prophetic with regard to some of his later work. (''Slow Train Coming'', anyone?)
* Music/{{U2}}'s "Peace on Earth":
-->''They say that what you mock\\
Will surely overtake you\\
And you become a monster\\
So the monster will not break you''
* Music/{{Radiohead}}'s "Bangers and Mash":
-->''If you stare into the dark, the dark will stare back\\
back into your SOOOOOOOOOUUUUUUUUUUUUUL''

* Music/{{Atreyu}}'s "[[http://www.youtube.com/watch?v=1FCyvV5V8Jo Becoming The Bull]]"
-->''Grab the bull by the horns the old adage goes\\
Nobody tells you where to go from there\\
Seems like fate's pulling you\\
Decisions have to be made\\
The best path is the hardest earned\\
Back and forth the struggle consumes us all\\
Trying to keep a level head\\
In the most unsettling of times\\
Today I'll become the bull.''

* This happens at the end of "[[http://www.youtube.com/watch?v=oxsFtCSOJyc Strength of the World]]" by Music/AvengedSevenfold after the narrator went on a RoaringRampageOfRevenge on those who killed his family:
-->''So far forever now alone, a greater punishment on me has been imposed\\
A killer falling from the light, I'll miss my family, I'll never be alright''
* "[[http://www.youtube.com/watch?v=zK268TLKCK4 Night of the Hunter]]" by [[Music/ThirtySecondsToMars 30 Seconds To Mars]]:
-->''I was born of the womb of a poisonous spell\\
Beaten and broken and chased from the land\\
But I rise up above it, high up above it and see\\
I was hung from the tree made of tongues of the weak\\
The branches, the bones of the liars, the thieves\\
Rise up above it, high up above it and see\\
Pray to your God, open your heart\\
Whatever you do, don't be afraid of the dark\\
Cover your eyes, the devil's inside\\
One night of the hunter\\
One day I will get revenge\\
One night to remember\\
One day it'll all just end, oh\\
Blessed by a bitch from a bastard's seed\\
Pleasure to meet you, prepare to bleed\\
Rise, I'll rise, I'll rise\\
Skinned her alive, ripped her apart\\
Scattered her ashes, buried her heart\\
Rise up above it, high up above it and see''
* "[[http://www.youtube.com/watch?v=HDKBeSMEBlk Murder]]" by Music/WithinTemptation and indeed most of the album ''The Unforgiving'' [[KnightTemplar is about violently hunting down monsters]]:
-->''I'm about to do it your way\\
I will make your world unsafe\\
I never thought you'd get this far\\
It's insane''
* Possibly implied in "Crawl" by Music/BreakingBenjamin:
--> ''[[IAmAMonster I'm becoming a monster,]][[NotSoDifferent just like you]]''
--> ''[[BreakTheBadass After it all, you'll try to break me too]]''

[[/folder]]

[[folder:Mythology]]
* In JapaneseMythology, a person who kills many {{Youkai}} will be transformed into a youkai. This occasionally gets played around with in games, manga, and anime from the country -- for example, in ''LaPucelle'', this is the basis for a NonStandardGameOver, one that gets taken more or less as canon in the ''Franchise/{{Disgaea}}'' series. And in ''Manga/InuYasha'', the murderous Bankotsu of the Band of Seven manages to transform his weapon into a demonic blade by using it to kill 1000 youkai and 1000 human warlords. It also shows up in ''Manga/{{Saiyuki}}'', where it's a part of Hakkai's backstory.
[[/folder]]

[[folder:Professional Wrestling]]
* [[Wrestling/StevieRichards Dr Stevie]] became more and more violent in his efforts to deal with the violently insane. In some ways, he was worse than his patients, as they really couldn't help it but Stevie knew exactly what he was doing. Then again, since Stevie thought this approach would work, all evidence to the contrary, one could say he had become insane himself, albeit in a different way.[[/folder]]

[[folder:Tabletop Games]]
* In ''TabletopGame/{{Warhammer 40000}}'', The Nighthaunter's backstory is made of this trope. Since he's the grimdark version of Franchise/{{Batman}}, what can you expect?
** Also: Radical Inquisitors, Commander Farsight some of the time...40K is the only game where even the ''monsters'' can succumb to this trope.
** The Imperium of Man itself can be this. It's a [[KnightTemplar theocratic]] {{dystopia}} that goes against the secular, rationalist utopia that their [[MessianicArchetype Emperor]] envisioned (The Emperor's Imperial Truth that promotes science and reason is now condemned as heresy, ''in his name''). Their Inquisition is apparently a power-hungry madhouse that is able to order the deaths of millions with a simple "just to be sure that there's no Chaos on the planet". Their armies are known for spreading death and destruction even to those they're helping, and will even fight amongst themselves if the situation calls for it. As a quote goes, "We used to pray for the Emperor to send us his angels. Now, we pray that he never does again."
*** Inquisitors must be sure that Exterminatus is the best and/or only option, as being baited by the Ruinous Powers into destroying your own worlds is, in fact, seen as just as bad as the world falling to Chaos. Indeed, there's at least [[DawnOfWar one]] canon example of a Chaos worshipper arranging for an Exterminatus ''on purpose'' so that he could dedicate the death of the planet and everything on it to the Dark Gods, an act for which he was elevated to Daemonhood.
*** When you gaze into the warp, the warp also gazes back into you...and laughs. "There is no peace amongst the stars, only an eternity of carnage and slaughter, and the laughter of thirsting gods."
*** Any inquisitor will eventually become this. There are two main sects of inquisitors, the Puritans and Radicals. Puritans believe that they must only destroy Daemons, never consort with them or use their wares (even a bolter touched by a daemon is considered tainted and must be destroyed). Radicals believe that they are allowed to use daemons against other daemons, so long as the purpose is pure. These two sects often have clashes and many of the Ordos are divided between these sects. However, all inquisitors start as Puritans, while almost all inquisitors die Radicals. This is because the WideEyedIdealist (and that's saying something, given the setting and [[MoralEventHorizon how they are willing to behave]]) that goes into the organisation slowly sees the futility of fighting alone, and slowly rationalise using daemons and daemonic weaponry against them. Most inquisitors are likewise more likely killed by their own peers than the daemons of the warp, usually by trial and execution at having found to harbor daemonic artifacts.
* The trope appears regularly in ''TabletopGame/DungeonsAndDragons,'' generally forming the backstory for antagonists, but sometimes providing players various forms of AntiHero to play.
** ''ForgottenRealms'' has an elven god, [[http://forgottenrealms.wikia.com/wiki/Shevarash Shevarash]], dedicated to nothing more than hatred and vengeance to Lolth and drow (though he readily beats anything AlwaysChaoticEvil in the absence of drow targets). Aside of other traits "endearing" to Seldarine and other "good guys", he's a little too close to the [[BigBad main, local, dark deity]] who makes Lolth look like a nice and caring mom by contrast. Elves who aren't immediately endangered and don't desire vengeance aren't too enthusiastic about his followers either. "The holy symbol of the faith is a broken arrowshaft that has been dipped in drow blood and blessed by a priest of Shevarash." (''Demihuman Deities'')
*** Another aspect is that while Shevarash himself considers other Underdark threats and theoreticaly is at peace with Eilistraee, his followers, obviously, seek vengeance for personal reasons -- and they swear to "never smile until the last drow is dead" without getting into more specifics. There even was a short story ''Necessary Sacrifices'' (in Realms of the Elves) about these fine People going over the top even in the eyes of a fresh convert.
** In the prologue of the rulebook ''Fiendish Codex II'', this is how Asmodeus transformed from a powerful angel to the ruler of all the devils. He started by fighting off the demon hordes, and began to willingly take on the traits of demons to better fight them. Although it could be argued that he was always evil from the start and the changes may have been solely physical in nature.
** The githyanki are so driven to exterminate their former slavemasters, the illithids, and prevent anyone else from gaining power over them, that they have become brutal conquerors. The other races are actually more afraid of the githyanki than they are of the illithids, since the illithids's power base is fairly limited. The githyanki even employ similar PsychicPowers. The gith who realized that this path would lead to He Who Fights Monsters broke away from the others long ago and became the more philosophical githzerai: they, too, regularly send 'pilgrimages' of killers to attack the illithids, but they have not fallen into CompleteMonster territory.
** The ''Heroes of Horror'' rulebook has the Corrupt Avenger prestige class, who is based on this trope, though their hatred is only for one group of monsters[=/=]creatures or organizations. Having abilities based upon [[TheCorruption taint]] further drives them to evil acts as they learn to fight their sworn foes.
** Third-party settingbook ''Shadows over Vathak'' has a feat called Dark Knowledge which grants the character the ability to prepare evil spells as bonus spells, but at the cost of taking health and Charisma damage when casting them. Its feat description states:
--> While the powers of the Old Ones bring death and decay, [[InvokedTrope sometimes it is necessary to use the weapons of evil to defeat evil]].
* Inverted with Dr. Rudolph van Richten of TabletopGame/{{Ravenloft}}, who started out his monster-hunting career determined to avenge his wife and son by any means necessary, but came to recognize the wrongness of this after he'd [[spoiler: sicced some zombies on a Vistani band and [[MyGodWhatHaveIDone watched them be torn to bits]]. Of course, given these were the Vistani who kidnapped his only son and sold him to a vampire to become the vampire's "bride" (a special form of spawn), and all because his efforts to treat one of their own wounded tribe members had failed and the badly injured Vistani had died, some could argue that [[KickTheSonOfABitch they had it coming]]. Still, that he is willing to realise that this is going too far is why he canonically has a Good-based CharacterAlignment]].
* In the ''TabletopGame/{{Pathfinder}}'' Adventure Path ''Second Darkness'', the Elven noble Allevrah Azrinae hunted the Drow with such passion and hatred that she ''turned into one'' (''[[OurElvesAreBetter Pathfinder]]'' [[OurElvesAreBetter Elves do that]]).
* ''TabletopGame/HunterTheVigil'' has this as an explicit risk of taking up the Vigil. Too many hunters have broken in the face of the world's supernatural threats and gone insane, becoming either maniacal killers or, worse, Slashers. The fiction for ''Slasher'' makes this explicit, as one story focuses on a hunter who started tracking down and killing mages because one of them killed his buddy; by the time he's working his way down to the ''relatives'' of mages, however, another group of hunters tracks him down and kills him.
** Similarly, in ''TabletopGame/HunterTheReckoning'', Hunters with very high Virtue ratings may become obsessed with that Virtue. Zealots (including Avengers, Defenders, and Judges) are particularly likely to become AxCrazy and tolerant of collateral damage.
** One of the subtler risks in ''TabletopGame/ChangelingTheLost'', for many reasons considered the ''bleakest'' of the NewWorldOfDarkness games. As a Changeling's Wyrd (power meter, roughly akin to, say, [[VampireTheRequiem Blood Potency]]) goes up, they remember their time in {{Arcadia}} more and more clearly. This is, in and of itself, frightening, but not dangerous. If their [[KarmaMeter Clarity]] hits zero, though...[[spoiler: they lose their grip on reality entirely and proceed to adopt the same BlueAndOrangeMorality as the True Fae, which is quickly followed by losing all traces of humanity and ''becoming'' a new member of TheFairFolk. In fact, this is the whole ''point'' of turning humans into changelings in the first place -- it's the only way the True Fae can '''reproduce'''.]]
*** Note that given the sandbox nature of the nWoD, this is the "point" of making Changelings only if the Storyteller wants it to be. True Fae are canonically capricious, selfish beings. They likely care nothing about the eventual consequences of their actions. That is simply the effect of the Wyrd of Arcadia on a human soul.
* In ''TabletopGame/LegendOfTheFiveRings'', the Kuni Witch Hunters are the people who hunt down people who have become tainted by oni/the Shadowlands. In search of these people and in search of how to destroy the Shadowlands, they frequently become tainted and summon Oni.
* ''TabletopGame/MageTheAscension'': The Euthanatos Tradition spends much of its time trying to clean the world of evil people that no longer serve their purpose in The Wheel, but in doing so expose themselves to a corruption known as [[TheCorruption Jhor]]. Jhor is gained by using magic to kill, killing unnecessarily, not maintaining a mundane identity with a job and friends so as to remain grounded, and so forth. People who succumb to Jhor [[EvilMakesYouUgly become pale and sallow, like they are ill]] and [[WithGreatPowerComesGreatInsanity far more extreme in measures, methods, and goals]]. They often [[WellIntentionedExtremist hold on to their original ideals and goals]] even as they continue to rationalize more drastic measures.
* In ''TabletopGame/{{Pandemonio}}'', this is part of the job description for Disciples -- you need to be willing to kill a possessed person to get at what's possessing them or sacrifice innocents for a boost if that's what it takes, or else you won't be much good at killing angels and demons. Taking this too far can invoke a literal version, however. One of the ways to raise [[TheCorruption Decay]] is to harm innocents, with a pass given to magics that harm them as a side effect of their use, such as many of the exorcism spells, or Maledictions -- the powerful magics necessary to fight some of the most powerful angels and have even a chance of winning, which are massively destructive by nature and often directly harm innocent bystanders as a side effect of use.
* Happens more than a few times in the course of ''TabletopGame/BattleTech.'' The most ironic example is probably Major Cullen Craig. He abandoned the mercenary regiment he was born into, the [[BraveScot Northwind Highlanders]], because they accepted a former Death Commando, Loren Jaffray, into their ranks due to a combination of his blood heritage descending from a renowned Highlander family and his actions that saved the regiment. Insisting that Jaffray's past as a Death Commando (widely considered the most ruthless and fanatical state-sponsored special operations group in the Inner Sphere) and a member of a Highlander family branch that did not return with the Highlanders to their native world, he did maintained that accepting Jaffray was a mistake and would lead to the unit's downfall. He joined Swann's Cavaliers to hopefully one day oppose the Highlanders and drive Jaffray out. So driven is he in his goal that he does not voice dissent when the Cavaliers then proceeded to join the Word of Blake, murder droves of civilians using their Battlemech companies in a massacre and in doing so earn massive bounties on ''all'' of their heads, and bring the fury of the entire elite 17th Recon Regiment onto them, who are now out for blood after the deaths of civilians on their home worlds. In a final fit of irony, the Highlanders never dismiss Jaffray, survive the bloody Word of Blake Jihad mostly intact, and make it out into the so called Dark Age setting as a highly respected, major faction unto themselves. On the other hand, Craig and Swann's Cavaliers, all wanted men marked for death, are never heard from again.
* In ''TabletopGame/{{Deadlands}}'', the validity of PayEvilUntoEvil allows for the existence of a fair number of "Gray Hats". Harrowed {[[RevenantZombie revenants]] timesharing their bodies with a demon that can [[EnemyWithin potentially take them over at any moment]]) are a character type available in all settings, whilst the classic game has several practitioners of BloodMagic (including a [[DefectorFromDecadence good-hearted rogue member]] of the resident clan of inbred {{Evil Sorcerer}}s) and the ''Hell on Earth'' game has prophets of radiation and mutation. That said, there are some characters considered so dark that it's inevitable that they ''will'' become monstrous {{NPC}}s in the end, with it being a question of how much good the player can do before that happens.
** The classic WeirdWest game has the Vampire character type, who are somehow more evil and inherently corrupt than the Harrowed.
** ''Hell on Earth'' has the Anti-Templars, who draw upon the power of the Reckoners and are invariably corrupted into Servitors as a result. They're mainly fit as player characters because most of them, at least initially, [[BadPowersGoodPeople only start taking the Reckoners' power so they can be strong enough to protect others]], and that's only because they disagree with the restrictive, oft-brutal code expected of the [[KnightTemplar Templars]].
[[/folder]]

[[folder:Theater]]
%%%* Aeschylus's ''[[Theatre/TheOresteia Oresteia]]'' trilogy.
%%%* ''Theatre/{{Hamlet}}'' is the most well-known play in the Revenge Tragedy genre.
* OlderThanSteam: ''TheRevengersTragedy'' starts when the Duke propositions Vindice's wife, and when she refuses, he kills her. And [[CrossesTheLineTwice rapes her corpse]]. And what Vindice does, in the course of getting his revenge...is actually a hundred times worse.
%%%* ''Theatre/TitusAndronicus''. Another Shakespearean side-splitter.
* Seymour, the shmuck who feeds the man eating plant in Theatre/LittleShopOfHorrors, was actually the monster himself. All the plant did was sit there and tell him what to do, until Seymour broke his promise, at which point it repossessed Audrey.
* ''{{Theatre/Sweeney Todd|TheDemonBarberOfFleetStreet}}'' is like the film example...only worse. [[spoiler:Johanna and Anthony Hope discover the carnage, with Toby turning the meat grinder, having gone completely insane. In some productions, Johanna and Anthony are under suspicion for killing the asylum keeper, and the blame for all of the murders falls upon Toby. In some shows, Todd opens his collar to allow Toby to kill him...]]
* CyranoDeBergerac: when Cyrano gives us his BullyHunter speech in Act I Scene IV, he has just bullied a poor Bore who only slightly bothered Cyrano...So Cyrano, according to the LawOfDisproportionateResponse, invokes the BerserkButton of his nose so he can dispense DisproportionateRetribution, kicking the Bore’s ass. It’s obvious that Cyrano has been bullied before [[TheGrotesque because of his enormous nose]], but he is so BadAss compared to anyone else in the play that now ''he is the bully.''
* In ''Theatre/TheSecretOfSherlockHolmes'', the titular secret is that [[spoiler:SherlockHolmes is actually [[DiabolicalMastermind Professor Moriarty]] as well, an alter-ego [[WellIntentionedExtremist to allow him a tight watch on the criminal world]] and [[GoodIsNotNice to keep him from getting bored.]]]]
[[/folder]]

[[folder:Video Games]]
* Creator/SamuelLJackson's character Officer Frank Tenpenny in ''VideoGame/GrandTheftAutoSanAndreas''.
* King Kashue will quote this word for word to the Trestkon towards the end of the [=WorldCorp=] ending in ''TheNamelessMod.''
* ''[[VideoGame/TwentyTwentySeven 2027]]'': One of the possible outcomes in the epilogue is that the Judicians end up becoming those whom they have fought.
* Ryu from ''Franchise/StreetFighter''. In the ''[[VideoGame/StreetFighterAlpha Alpha]]'' series and beyond, this trope is a problem that he is constantly wrestling with. His fighting style has an inherent dark side called the Satsui no Hado (Surge of Murderous Intent) that he tries to contain, while his nemesis, Akuma, repeatedly tries to force Ryu to give in to it.
* It's probably appropriate, given that they turn into monsters, that the Harmonixers of ''ShadowHearts'' face this. The downside to their MegaManning is that in addition to taking in the powers of those they slay, they also take in their hatred, sorrow, and anger. This can have consequences including death, madness and souls being twisted to evil, or becoming feral beasts that exist only to destroy.
* Prince Arthas in ''VideoGame/{{Warcraft}} III''. This was apparently engineered (or at least taken advantage of) by the BigBad, to the point of sacrificing Arthas's target so that the fall could be complete.
** The box for ''Wrath of the Lich King'' even reads "If you stare long into the abyss... the abyss stares back into you."
** The ''VideoGame/{{Warcraft}}'' series uses this one a lot. The only villains in the series (including the Big Bad, Sargeras) that haven't been driven mad by endlessly fighting monsters are the [[Creator/HPLovecraft Lovecraft]]-inspired Old Gods, the demons Sargeras corrupted and forced into his [[DemonicInvaders Burning Legion]], and the Naga, who were made by the Old Gods (they did not even have a choice!).
** Clearly lampshade-hung by Malfurion regarding Maiev in ''Warcraft 3'', when he says that she has become "vengeance itself" and hopes that, in her pursuit of Illidan, she will not wreak even more havoc than him. But by the end game of the ''Burning Crusade'' expansion, Maiev says that she is '[[BadassDecay indeed nothing]]' after downing Illidan.
*** In the expanded universe novel "Wolfheart", Maiev returns, disgusted at the night elves having become so reliant on outsiders as well as allowing the Highborn to return to their society, and attacks Malfurion and Tyrande. Malfurion, who may have been still a tad bitter Maiev finally caught up to Illidan, lampshades this as she's fleeing when her attempt fails.
--> '''Malfurion''': "Who's the traitor now, Maiev?"
** Illidan himself qualifies after consuming demonic powers to fight the Burning Legion. After being banished by his brother, he briefly works for the Legion in bringing down the Lich King. Failing that, he retreats to Outland to escape their wrath. The evil magic he consumed, which also fills the very air of Outland, drives him mad between VideoGame/{{Warcraft}} III and VideoGame/WorldOfWarcraft.
** After the death of the Lich King, [[spoiler:Sylvanas]] seems hell-bent on becoming the leader of a new Scourge by is using the displaced Val'kyr to raise the dead to repopulate the Forsaken. Even ''Garrosh'' points out how close [[spoiler:she]] is to becoming like the Lich King.
** The [[KnightTemplar Scarlet Crusade]], the fanatical undead-hating organization, which has made the exact same journey as the Lich King/Arthas, have, as of Cataclysm, [[spoiler:become undead themselves, because of Balnazzar's VillainousBreakdown ]]
*** Same can be said for the renegade group of Scarlet Crusaders that Alliance players assist. They succeed in purging the Scarlet Monastery of its corrupted members but ultimately succumb to insanity after killing their own men.
** Fandral Staghelm seems to have come full circle on this. [[spoiler:During the War of the Shifting Sands, his son, Valstann, was killed in front of him by the Old God-aligned Qiraji, which he [[DespairEventHorizon never fully got over]]. He went to any length to bring his son back to life, even if it meant corrupting the night elves's world tree, Teldrassil. An Old God agent, posing as Valstann, completely shattered his illusion and the remnants of his sanity. By the time of the Cataclysm, he commands the Druids of the Flame, a breakaway sect aligned to Ragnaros the Firelord - who himself answers to the Old Gods.]]
** A particularly jarring example occurred in Mists of Pandaria, where [[spoiler: Jaina Proudmoore, after being adamant about desiring peace between the Alliance and Horde, abandoned her ideals and became what is essentially a female version of her father after Garrosh destroyed her city Theramore (worth noting that she had originally subverted this in the novel Tides of War, when she realized that her desire to destroy Orgrimmar with the Focusing Iris in retaliation made her no better than Garrosh). Even Varian Wrynn himself noted how shocked he was at the whole thing.]]
* The opening quote of the beginning cut scene of ''BaldursGate''.
** And ''VideoGame/{{Diablo}}''.
*** And ''TooHuman'', which even gets its title from another Nietzsche work, ''Human, All Too Human''.
**** And ''Legends of Valour''.
** ''BaldursGate'' is actually a subversion, since, while it is certainly possible to gradually turn to evil in your war against Sarevok, the game strongly rewards you for being good.
* In ''VideoGame/PuzzleQuest'', the main character falls into this in the bad ending, effectively replacing the villain of the story.
* The video game ''PhantomBrave'' features a character like this. His name is Sprout, and he knew that he was absorbing the BigBad the more and more pieces of it he killed. He thought that he could control it, and when he couldn't, he killed himself and the parts of the BigBad that were fusing with him. This HeroicSacrifice weakened the BigBad for the heroes to fight.
* In Disgaea: Hour of Darkness, main villain Archangel Vulcanus believes demons are pure evil and angels are pure good (untrue in Disgaea) and seeks to destroy the demons. This blinds him to his own evil actions, [[spoiler:including provoking war between Earth and the Underworld and enlisting demon assassins]].
* In ''VideoGame/KnightsOfTheOldRepublic'', we are told that this is what happened to the Jedi Knights Revan and Malak during the Mandalorian Wars prior to the first game. Kreia explicitly states as much in the second game: "As Revan and Malak fought the Mandalorians in battle after battle, they grew to despise weakness, just as the Mandalorians did. In the end, the Mandalorians had taught them through conflict. Shaped the Jedi."
** We also get a bit of {{foreshadowing}} on Bastila when she's in her full on KnightTemplar mode, talking about how a Jedi must "harden their hearts" and "do what is necessary" to fight the Order's enemies.
** And [[VideoGame/StarWarsTheOldRepublic 300 years later]]: [[spoiler: The newly-freed and still-alive Revan (''very'' long story) decides the best way to pay back the Sith Emperor for that AndIMustScream imprisonment is by coming up with an army of {{KillerRobot}}s that will kill ''anyone'' with even a trace of Sith blood...roughly 97% of the Empire's citizens and quite possibly a few trillion Republic lives as well.]] So much for spending all that time maxing out the blue side of your KarmaMeter on the first two games, and NiceJobBreakingItHero for the Republic players...
* In ''VideoGame/FinalFantasyIV'', the elder of Mysidia uses this phrase to warn Cecil that he could fall into this trap until he willingly sheds his darkness.
* NipponIchi occasionally does this straight (particularly with [[spoiler: [[VideoGame/Disgaea3AbsenceOfJustice Aurum]]]]), but is more often likely to apply this trope ''literally'' -- several characters have wound up as demons simply by killing a lot of them.
** Special mention goes to Prier of LaPucelle, [[spoiler: because this is her ''canonical ending''.]]
* Baby Bonnie Hood/Bulleta from ''VideoGame/{{Darkstalkers}}'' let her heart become so blackened in her quest to kill all the paranormal creatures that the BigBad of the third game thought that she was a powerful Darkstalker (i.e Demon) and summoned her to Majigen to fight in his tournament. She failed to notice this, though, and just said:
-->"Heh, they're all mine... it's been a long time since I had a job this big."
** The [[AllThereInTheManual official]] [[{{Canon}} sources]] say that she was actually born with a dark soul which makes her resemble Darkstalkers.
* In Terminal Reality's awesome monster hunting game ''VideoGame/{{Nocturne}}'', the protagonist, the Stranger, straddles the line of this trope without really going over it. He has an intense hatred of literal monsters, but he will begrudgingly work with them when the situation calls for it, such as by getting inside information from the [[FrankensteinsMonster reanimated mobster Icepick]] or working side-by-side with the [[HalfHumanHybrid dhampir]] Spookhouse agent Svetlana Lupescu. On the other hand, the retired Spookhouse agent Hamilton Killian vaults over this trope at a running pace with a pole: when his wife was [[TheVirus infected by a vampire,]] causing both herself and their unborn child to become undead, Hamilton, already a monster killing machine, went completely over the edge, unable to tolerate the presence of even ostensibly good ?monsters?. In the game, he sinks to the point of [[spoiler: trapping the Stranger in his estate, subjecting him to a series of lethal traps and captured monsters. His justification? The Stranger and Spookhouse in general have obviously become ?monster lovers? for working alongside individuals like Svetlana and Icepick, rather than mercilessly killing them.]]
* Parodied in ''VideoGame/Left4Dead'': the graffiti in one safehouse reads "WE ARE THE MONSTERS" in large letters, and has a few scathing replies underneath it, including: "No, that would be the zombies", "Have you even looked ''OUTSIDE''", and "I hope you are dead now."
** "I miss the internet."
* In ''VideoGame/{{Fallout 3}}'', a costumed male character calling himself "The Mechanist" creates a robot army to defend the town of Canterbury Commons from the "[=AntAgonizer=]", a costumed female character leading an army of giant mutant ants. The inhabitants of Canterbury Commons tell the player character that the Mechanist's laser-wielding robots are more of a threat to the town than the ant army ever was.
** Although that's more because the [=AntAgonizer=] is a laughable HarmlessVillain that the townsfolk can deal with just fine by themselves, rather than any real fault on the Mechanist's part (well, he ''is'' a delusional "superhero", but what costumed crimefighter isn't?). If Canterbury Commons was up against Raiders or Enclave, the Mechanist's ability to crank out gatling laser-wielding robot soldiers would probably be better appreciated.
* In ''VideoGame/{{Penumbra}}: Black Plague'', after learning a big lesson about the Tuurngait being [[spoiler: the original inhabitants of earth who liked mankind but hid because mankind was becoming violent,]] and completing some trials to show that [[spoiler: you understand that the Tuurngait are not evil,]] you contact the outside world and [[spoiler: reveal the location of the mine, and request that they KillThemAll.]] Then again, we never get proof that they aren't ''lying''...
* The Church in ''{{Tsukihime}}'' is known as being fanatically zealous about killing non humans. Other characters imply, and we have even seen, that there are some demon hybrids/vampires who are neutral or good. The Church does not care, so it does not tend to get along with those like the Tohnos (hybrids, but avoid inversion), Arcueid (vampire, but kills vampires and is too tough for them to beat), and the Nanayas, who only targeted the Inverted. Notably, the only Church characters we see are a bit heretical for not blowing everything up first and then covering it up.
** It's not surprising for them to be heretics considering that the Church is repeatedly stated to have no influence in Japan--since any members of the Church are the only representatives of it for dozens, if not hundreds, of miles, it's pretty safe to act as they please.
* [[ArmiesAreEvil Blackwatch]] from ''VideoGame/{{Prototype}}'' is about as evil as they come, fighting a merciless viral threat that doesn't care for the laws of war or human rights. Conversely, our player character Alex Mercer starts off real sociopathic and very slowly develops a conscience. Mostly, it is implied, by consuming people who have one.
* In ''ShadowOfTheColossus'', you [[spoiler: end up absorbing the colossus essences, culminating in becoming a demon yourself]]. Granted, later on, you [[spoiler: are reborn as a baby, but still...]]
* ''VideoGame/TalesOfVesperia''. The guild-wide FreudianExcuse doesn't excuse the Hunting Blades from acting worse than the "monsters" they hate so much.
** Yuri himself skirts this trope and is well aware that he almost falls into this. Ironically the one who ''did'' fall into this is [[spoiler: Sodia. Her own VigilanteExecution of Yuri basically made her into what she had always despised him for.]]
* In ''[[SoulSeries Soul Calibur IV]]'', Maxi has let his hatred of Asteroth consume him to such a degree that he's willing to use the soul-consuming sword his friends have been desperately trying to destroy to fight him. [[spoiler: In his ending, he's even willing to take up the sword against Asteroth with the knowledge that his friends will have to fight him afterwards.]]
* By the time of ''VideoGame/MegaManZero'', ''VideoGame/MegaManX'' admitted to no longer caring about the enemies he fought. What's worse was that if it weren't for the ContinuitySnarl between the two series, he [[spoiler:would've become a full-out Maverick KnightTemplar...]] X, however, was savvy enough to realize this and took himself out of the fight [[spoiler: by acting as a living seal for the Dark Elf]] before he could go over the edge.
** [[BigBad Dr. Weil]] is a straighter example. For all the acts of revenge he pulls on both humanity and the Reploids, he completely ignores the fact that he has become what he hates the most: a Maverick, in all definitions of the word in that universe.
** In ''VideoGame/MegaManZX'', the manner in which the ArtifactOfDoom, Model W, sows conflict and destruction in the world - and in doing so, grows stronger - exists at the point of intersection of this trope with TheVirus. If fighting monsters (both literal and metaphorical) ultimately causes you to become a monster - with Model W [[MoreThanMindControl accelerating the process]] - then those who fight ''you'' will ''also'' become monsters; apply recursively. This never-ending cycle is explicitly pointed out late in the second game. Appropriate, given who Model W used to be...
* ShadowComplex flirts with this in the subtext. The main character notes that killing has become easier for him, and he even gains a suit of the very same PowerArmor the enemies use, making him identical in appearance to them.
* Samus Aran of the ''{{Metroid}}'' universe almost succumbs to this on a couple of occasions.
** The 3rd chapter of the manga describes an early recon mission she has as a [[CowboyCop Galactic Federation Police Officer]] on the planet of Jigrad. She and two of her squad mates save a group of slaves from the Space Pirates. When confronted by Samus, the last remaining Space Pirate pleads with her to show mercy. Driven by her memories of her homeworld K-2L being mercilessly razed by the Space Pirates, Samus is about to murder the last Space Pirate in cold blood when the sound of a small girl crying snaps her out of her rage. Shortly thereafter, the small child thanks Samus for saving her, and Samus tells her that she saved her as well from becoming just as much of a merciless killer as the Space Pirates.
** Also the ending of Metroid II. Samus is charged with the task of eradicating the Metroids once and for all. After destroying the Metroid Queen, [[spoiler:Samus finds a newly hatched Metroid. Despite knowing that Metroid is a Chozo word for "ultimate warrior", she still can't bring herself to kill a child.]]
* ''Franchise/MassEffect'' has [[CowboyCop Garrus Vakarian]] do this. He originally quit C-Sec to join Shepard's team because of all the red tape, as he says that it shouldn't matter how he got the job done as long as he did it. In the first game, his personal mission involves finding an organ-harvesting doctor who got away from him - Garrus's first instinct is to kill him on the spot ("I'd harvest your organs first, but we don't have the time.") Shepard can either encourage him or convince him to at least try to take him alive.
** In the [[VideoGame/MassEffect2 sequel]], his loyalty mission has him tracking down the only member of his vigilante squad to live through an ambush (considering that he was a {{Turncoat}}), wanting to kill him. Shepard has the option to cooperate or block Garrus's shot and have the guy tell him that he's a dead man walking.
** Liara almost does this as well as her quest for revenge on the Shadow Broker for [[BerserkButton trying to hurt Shepard]] causes her to [[BreakTheCutie turn from a sweet and innocent archaeologist to a ruthless information broker]] who barely trusts anyone and threatens people in the same way her mind-controlled mother did two years before. She even alienates Shepard, [[UndyingLoyalty the one person she risked everything for]], when s/he finally comes back from the dead. Ultimately she eventually averts this...kind of. [[spoiler:After killing the Shadow Broker, she becomes the Shadow Broker but vows to use the information to help Shepard fight [[EldritchAbomination the Reapers]] and will use that motivation to keep herself honest.]]
** [[EldritchAbomination The Reapers]] have this as one of their special powers. Indoctrination is an effect that slowly alters the minds of everyone in their vicinity to be more compliant towards their suggestions and assume their way of thinking. This effect even persists if they are dead, making even studying their remains dangerous. It seems very likely that [[spoiler:this happened to the Illusive Man who studies the Reapers for years and eventually thought it was a good idea to implant reaper technology into his own body, which caused him to simultanously fight the Reapers and attempting to do the very same thing he wants to prevent them from doing.]]
** ''Shepard'' can go this route if you play a strict Renegade.
* ''InjusticeGodsAmongUs'' applies this to the heroes of the DC Universe: in an alternate timeline, after killing the Joker for destroying Metropolis and causing the death of Lois Lane, Superman goes on to establish a dictatorship with the goal of not allowing what happened to Metropolis to ever happen again. Eventually, this desire to protect becomes twisted into a desire to subjugate and conquer: part of his plan after crossing the MoralEventHorizon is to cross over into the main DC timeline, conquer its earth, and take its Lois as his bride.
* ''SuikodenTierkreis'' makes this recursive. You can take down the BigBad the easy way by sacrificing the lives of all your allies, or look for another, harder way to beat him. [[spoiler:On the hard path, you find a copy of the tablet that lists the names and magical affinities of all your allies. Every name on it except the one under the "[[StandardizedLeader Tenkai Star]]" has been burned off, since they sacrificed their lives to stop a villain before this one, except for the leader who became the fellow you're now fighting. On the easy path, [[ShootTheShaggyDog you replace him.]]]]
* In ''Franchise/DragonAge'', the Grey Wardens are an order of warriors, rogues, and mages dedicated to fighting [[OurOrcsAreDifferent the darkspawn]]. To do so, they ingest darkspawn blood, making them more powerful and allowing them to sense the darkspawn. This also slowly turns them into darkspawn like creatures. The Wardens are also known for their ruthlessness in fighting the darkspawn, as they have no qualms against killing people who have been infected with the darkspawn taint, although they are nowhere near as brutal as the darkspawn.
** Loghain Mac Tir is the main antagonist of the game, and is so paranoid that he refuses to ask for help when Ferelden is on the verge of defeat, and sparks a civil war, simply because the king was going to ask for help from the Orlesian Empire, which had invaded Ferelden years ago. As such, he leaves the king to die in the opening battle, because he's paranoid that it's handing Ferelden back over to the Orlesian Empire, when the entire country is about to be destroyed. In the process of this all, he becomes everything that he hates, and never realizes it--some Ferelden citizens will speak in hushed tones that he now resembles the very Orlesians he once threw out of the country.
** In order to fight the darkspawn, the dwarven smith Caridin created an Anvil that allowed him to [[spoiler: painfully transform dwarves into golems. He stuck to volunteers at first, but the king ordered him to start using the poor, criminals, and the king's political enemies. Eventually, Caridin realized what [[MyGodWhatHaveIDone he and the king had become]] and refused to do any more of this, and the king ordered Caridin's apprentices to turn him into one.]]
*** Branka -- was she a monster before, or did she become one due to her obsession with the Anvil? The epilogue shows that anyone who gets hold of the anvil becomes pretty inhuman (indwarven?) with its use. To get it, Branka went that extra step and mutated her own people, who hunted monsters with her, involuntarily into monsters by allowing them to be raped and forced to eat the flesh of their friends. These monsters give birth to the monsters they hunted and Branka used them to set off the traps guarding the anvil.
* In ''VideoGame/DragonAgeII'', Anders has a very literal case of this as he has voluntarily allowed the spirit of Justice from Awakening to possess him, but his anger has warped Justice into a force of vengeance against the Templars, and Anders has become very much a WellIntentionedExtremist.
** To be honest, the entire storyline is built on this. [[spoiler:The leaders of both factions eventually give into their demons. Knight-Commander Meredith, who had fought blood mages all her life (okay, and an ArtifactOfDoom), calls for the Rite of Annulment ordering the death of every mage in the city for the action of one rogue mage; and First Enchanter Orsino, who resorts to BloodMagic to put down Meredith and the templars who had oppressed them. They both [[JumpingOffTheSlipperySlope jump off the slippery slope]] for what they see as the [[WellIntentionedExtremist right reason]]. The major irony is that they both ignore the most important flaw in their own factions; Orsino allows a serial killer mage to go unchecked because he went so far that he broke new ground, and Meredith uses dangerous anti-magical techniques to instill absolute order - including demonic possession, and end up literally becoming monsters that are the opposite of their class; Orsino uses blood magic to become a hulking corpse bruiser with a slippery-quick center (Reaver and Assassin), and Meredith uses a cursed sword in the name of the Maker, not realizing that she just used magic to jump twenty feet in the air and re-activate the city's antipersonnel robots (Battlemage and Demon Summoner).]] It really fuels the game's relentless GreyAndGrayMorality.
* Considering the back story of ''{{Killzone}}'', the Helghast could be seen as these. Fleeing to a DeathWorld after losing a war trying to resist the [=UCN=]'s tyranny, they became as bad as the very people they fought.
* In ''VideoGame/BlazBlue'', Kokonoe has a seething hatred of Terumi Yuuki. In her attempts to bring him down, though, she has committed some acts [[MoralEventHorizon that are earning her the ire of the fanbase,]] like treating [[TheWoobie Lambda-11]] as if she's nothing but an inanimate tool. She is aware of this fact (doesn't even try to cover it up with justifications) and notes to herself that she has crossed the point of no return a long time ago.
* Maxim from ''VideoGame/LufiaIIRiseOfTheSinistrals'' starts off as this, but quickly evolves into a more standard hero.
* The Patriots in the ''Franchise/MetalGear'' series is a good example of such an issue: They were initially formed to essentially eliminate the Philosophers and replace it with The Boss's teachings. Unfortunately, soon after that, Zero, Para-Medic, and Sigint ended up becoming corrupt, and eventually became the very thing that they attempted to remove.
* ''VideoGame/DiabloIII'':
** Averted by the playable Demon Hunter, who is utterly ruthless in their fight against all things demonic, yet doesn't let it compromise their goal of protecting the innocent. This isn't a given for Demon Hunters in general, however. The short story ''[[http://us.battle.net/d3/en/game/lore/short-story/demon-hunter/1#read Hatred and Discipline]]'' describes it as every Demon Hunter having to navigate the treshold between good and evil, it being all too easy to lose control over their fear and hatred, and "cross over to the other side".
** Tyrael invokes this in regards to Zoltun Kulle, whose efforts to create the Black Soulstone drove him to murderous megalomania.
** Malthiel fell headfirst into this in the expansion pack, Reaper of Souls.
** Referenced in the FlavorText from the legendary sword ''Monster Hunter'': "Be wary when you fight monsters, lest you become one."
* ''VideoGame/{{Tekken}} 6'''s Jin Kazama quotes Nietzche word for word after his final encounter with Lars. As far as Jin is concerned, however, he will do whatever it takes to end his bloodline, including becoming just as evil/hated as Azazel, who was responsible for most of the conflict in the Mishima bloodline.
* ''VideoGame/CityOfHeroes'' allows the player to fall from a Hero to a Vigilante by taking some morally dubious actions, and eventually fall to Villain. Fortunately, you can pull a HeelFaceTurn.
* Subverted in ''VideoGame/{{Touhou}} 12'' with the origin story of Byakuren Hijiri, a youkai hunter who came to genuinely sympathize with youkai and eventually converted herself into one. In the same game, Sanae Kochiya is warned that if she continues to hunt youkai with such zeal, she will play this trope straight. There has been mention of [[CuteWitch Marisa Kirisame]] possibly converting herself into a youkai in a bid for power, though it hasn't happened.
* Happened to [[spoiler: Fain and his followers]] during the [[HopelessWar Elder Wars]] in ''{{Lusternia}}''. They start out employing methods that the other Elders found [[KnightTemplar questionable]], and end up JumpingOffTheSlipperySlope and ''eating'' their fellow Elders to [[CannibalismSuperpower fuel the war effort]] - which happens to be the [[NotSoDifferent same threat]] presented by their foes, The Soulless.
* It's lightly hinted at in ''VideoGame/KingdomHeartsII'' that [[TheHero Sora]] is slowly falling into this, based on how much more aggressive, ruthless, and rude he is toward the enemies than he was in the first game. Then again, fighting the forces of Darkness themselves can take its toll on people, especially when you have [[spoiler: dozens of people inside your soul]].
** WordOfGod says this outright, also stating that Anti-Form is a product of it.
*** [[spoiler: In ''KingdomHearts3DDreamDropDistance'', Sora finally succumbs to the darkness in his heart, and nearly becomes a [[{{GrandTheftMe}} Xehanort clone]]. Even worse is that he is the FinalBoss of the game. He gets better, thanks to Riku]].
** [=DiZ=][=/=][[spoiler:Ansem the Wise]], who, in his efforts to make sure that neither TheHeartless nor the Organization XIII takes over the universe, rely on kidnapping, psychological manipulation, and playing the YouHaveOutlivedYourUsefulness card on his Nobody allies. At least he regrets it in the end before he [[spoiler:got blown up and killed, [[DisneyDeath for a while.]]]]
* In ''VideoGame/{{Nier}}'', the protagonist develops an all-consuming hatred for the Shades after The Shadowlord kidnaps his daughter at the end of the first half of the game. It's really shown in the NewGamePlus where [[spoiler:the players can now understand the language of the Shades, AKA the souls of the true humans, and see the backstory of the bosses you've been mercilessly cutting down. Nier is Robert Neville.]]
* ''VideoGame/SpecOpsTheLine'' - Walker becomes increasingly violent, and commits unmitigated ''war crimes'' in his pursuit to kill John Konrad, the man who rules the ruined city of Dubai. [[spoiler: However, Walker does [[MyGodWhatHaveIDone realize what he has done]], through flashbacks and hallucinations]]
* In ''VideoGame/{{Syndicate}}'' (2012): The CIA weren't quite heroes in the first place, but they were one of the last holdouts of governmental force against the syndicates. However, with funding at critical levels, they turned into the Independent Intelligence Agency, which is itself a syndicate. The infobank entry directly mentions becoming the monster they used to fight.
* ''SinsOfASolarEmpire'': The Rebel TEC want revenge on the Advent, and Vasari by striking back, and being as ruthless as they were, their Titan ship emphasizes this as its all about attacking, its a space gun with [[MoreDakka guns all over it]].
* In a very literal case, in [[DarkSouls Dark Souls]], the Knight Artorias clashed with the dark monsters of the Abyss, earning the title of Knight Artorias the Abysswalker. [[spoiler: Unfortunately, he was eventually overcome by the King of the Abyss, Manus, becoming the very kind of twisted creature he fought for so long.]]
* In ''VideoGame/BioShockInfinite'', the [[LaResistance Vox Populi]] purport to be noble freedom fighters who oppose the violently racist/fascist Founders who rule over Columbia. Years of fruitless fighting, though, have gradually caused them to degenerate into a band of bloodthirsty marauders who fight out of sheer blind hatred for their enemies. Their leader's first move upon gaining power is to shoot and scalp a factory manager (who, admittedly, was a completely evil SOB) in cold blood... before attempting to kill an innocent child on the grounds that they might ''possibly'' grow up to be as bad as their father. Even after [[spoiler:Elizabeth kills her]], the Vox continue their rampage throughout the game's climax, murdering random civilians left and right while burning the city to the ground and loudly proclaiming that everything now belongs to them.
* In ''VideoGame/CastlevaniaLordsOfShadow'', Gabriel Belmont is a knight of the Brotherhood of Light, charged to protect and defend innocent mortals against monsters. His quest revolves around trying to resurrect his murdered wife, Marie. Towards the end of the game, he learns upon confronting the Lord of the Necromancers, [[spoiler:Zobek]], that he has a dark side within him so terrible that the Lord of the Necromancers found it surprisingly easy to control him in his sleep and [[spoiler:kill Marie]]. The main game ends with Gabriel given a chance for redemption, but the DownloadableContent reveals that during his quest to contain the Forgotten One, [[spoiler:he is forced to become a vampire in order to enter the Forgotten One's prision, and gets corrupted by the Forgotten One's power upon claiming it for himself]]. By the time TheStinger rolls around, [[spoiler:Gabriel has a ThatManIsDead attitude and proclaims "Eu sunt {{Dracul|a}}!"]]
* By the end of ''VideoGame/SaintsRowTheThird'', STAG has lost any moral high ground it might have had, having [[spoiler:declared martial law, carried out abduction and detention without trial (on Shaundi), rigged a major monument to blow in a terrorist-like manner]] and, in one ending, trying to [[spoiler:level the whole city]] to get at the Saints. In the other ending, the one who sent them hangs a lampshade on this, pointing out that our favourite {{Villain Protagonist}}s acted more heroically and asking who the people are going to support after that.
** By ''VideoGame/SaintsRowIV'', [[spoiler:Cyrus Temple, the leader of STAG, has gone from fighting monsters to becoming one, forming a terrorist cell in the Middle East with the intent of destroying Washington DC on the basis that the Saints "ruined America".]]

* ''VideoGame/TheWonderful101'' [[spoiler: The GEATHJERK are those that fight monsters: They came from 1,500 years in the future to destroy Earth because in their time, Earth has become an Evil Space Dictatorship, using Wonderful technology, and invaded their planet, and they try to destroy Earth... by becoming an Evil Space Dictatorship and using Wonderful tech in the final boss battle. It all becomes 1 giant Timey Wimey Ball once you consider Earth probably invaded them as revenge for invading us in the present.]]
* ''VideoGame/{{RefleX}}'' features [[spoiler:the Phoenix / ZODIAC Ophiuchus]], which is so bent on destroying the hostile ZODIAC units, which humans describe as "Winged Menaces", that [[spoiler:it ends up unapologetically destroying a horrifying percentage of Earth and humanity, causing it to be labeled as simply yet another winged menace.]]
* In, ''VideoGame/DwarfFortress'', the players will often be forced to resort to rather extreme solutions to survive some of the most dangerous events and monsters. That being said, the community being what it is, those events and monsters are less of a cause and more of a convenient excuse for the methods.
[[/folder]]

[[folder:Visual Novels]]
* ''VisualNovel/AceAttorney'''s [[spoiler:Godot]] definitely qualifies. Starting as a [[spoiler: defense attorney, he gets poisoned by a criminal, barely escaping death with damaged sight. Upon finding that his girlfriend, Mia, was killed by a criminal, and Phoenix failed to help her, he holds a grudge against him, and sets up ThePlan to prevent Maya, Mia's sister, from suffering the same fate...only to find the criminal that poisoned him, Dahlia, and in his rage, try to kill her.]] While he was successful, he admits that he acted out of revenge rather than the desire for justice.
** Even [[spoiler:Phoenix Wright]] himself becomes this in Apollo Justice. After having his attorney's badge taken away for [[spoiler:presenting forged evidence, he loses respect for the whole judicial process, and manipulates crime scenes, forging evidence to get Kristoph Gavin convicted.]]
[[/folder]]

[[folder: Web Comics]]
* In ''Webcomic/TheAdventuresOfDrMcNinja'', when the doc is subject to MoreThanMindControl by an evil motorcycle helping him hunt down King Radical, this trope provides the "More Than" part.
-->'''Alt Text:''' To be fair, Doc always WANTED to blow up helicopters. The bike just gave him the means.
%%%* ''AmazingSuperPowers'' provides an [[http://www.amazingsuperpowers.com/2009/02/loose-cannon/ amusing variation]] of this.
* Parodied in ''AnsemRetort''.
-->'''Sora''': (From down in the abyss) Hi Riku!
-->'''Riku''': Damnit Sora! Get out of the [[SymbolSwearing %$#@^&]] abyss!
* ''BittersweetCandyBowl'': After years of taking {{Tsundere}}-ish abuse from Lucy, Mike finally, shall we say, "gets her back" by not only [[spoiler:turning her down, but eventually ''[[KickTheDog brutally and deliberately destroying]]'' any trace of their relationship, professional or otherwise]]. The problem is, [[spoiler:Lucy has already gone through ''extreme'' CharacterDevelopment by this point, so Mike's actions [[DespairEventHorizon cause her to spiral into a deep, near suicidal depression,]]]] [[TookALevelInJerkass which makes him as bad, if not WORSE to her than she ever was to him.]]
* ''Webcomic/{{Oglaf}}'' [[PlayedForLaughs plays this trope for laughs]] in that [[http://oglaf.com/abyss/1/ strip]].
* When the second part of the phrase (the abyss part) was used in ''Webcomic/TheBMovieComic'', creator Roman Wunderlich declared in TheRant that he didn't understand what the big deal was:
-->"I never understood what's supposed to be so bad about that... I mean, I'd see that the abyss is deep, and the abyss'd see that I'm shallow - but it's not like I've been denying that, anyway..."
* Jenn of ''Webcomic/CaseyAndAndy'' just manages to catch herself [[http://www.galactanet.com/comic/view.php?strip=416 doing this]].
* The Vatican and Aesir churches of ''CryHavoc'' level cities to destroy a handful of daemons. Although, seeing as the last time their foes congregated, a decade long war that killed half of the worlds population occurred, they may be more justified than most.
%%%* ''DominicDeegan'', with Karnak.
* This is invoked in ''Webcomic/GastroPhobia'': when Bambikles seeks to avenge his mother by killing [[HumansAreBastards the monster]] who took her life, he's told that "To ''kill'' a monster, you must ''become'' a monster." So he does.
* ''Webcomic/TheOrderOfTheStick'':
** [[spoiler: Vaarsuvius]] became a very literal example of this trope but has since "recovered" and now regrets their actions.
** Redcloak, depending on [[AlternateCharacterInterpretation your interpretation.]] Although in his case, it's less a matter of fighting with (against) his enemies and more fighting with (alongside) Xykon.
%%%* As is usual, ''PennyArcade'' gives us quite the [[http://www.penny-arcade.com/comic/2008/4/11/ atypical example.]]
* Professor Broadshoulders from ''ZebraGirl'' is obsessed with destroying demons and people tainted by demons, to the point where he sacrifices his very soul, giving into his own demonic curse and physically transforming into a demon, to destroy Sandra, the titular zebra girl, despite the fact that Sandra was still a good person, despite being transformed into a demon herself. Appropriately, it was Broadshoulders's attempts at destroying Sandra that finally pushed her over the edge, turning her away from [[IJustWantToBeNormal wanting to cure her condition]] to [[EvilTastesGood indulge her demonic hunger for pain and torment]].
* In ''Webcomic/{{Pacificators}}'', the reason regular people hates those with [[ElementalPowers powers]] so much is because one of their best Pacificators ever, [[SanitySlippage Castella]] [[DisproportionateRetribution Brandsford]], went mad and [[TearJerker murdered thousands of people]] before she was finally killed herself.
[[/folder]]

[[folder:Web Original]]
* In ''Roleplay/TheGamersAlliance'', [[LoveableRogue Refan]] becomes a more jaded and bloodthirsty killer as he ends up having to fight more and more enemies to protect his loved ones. When the enemies get tougher, he has to resort to using his demonic side, which is slowly corrupting him.
* The Dove, from the ''GlobalGuardiansPBEMUniverse'', started out as a standard street-level superhero who concentrated on finding and stopping serial killers. Ten years later, after he is arrested for the murder of his latest target, he suffers a HeroicBSOD when it is pointed out, finally, that he isn't a hero but rather is just a serial killer himself...one who targets other serial killers.
* Adam Dodd in ''SurvivalOfTheFittest'' turns into one of these for a good while during his tenure on the v1 island. His obsession with getting revenge on Cody Jenson leads him to mow down a good six or seven of his fellow students, despite his supposedly heroic motives. In something of a subversion, however, he lives to come to realise that his actions have been misguided and returns to a more conventional AntiHero mold.
** The entire concept of player-killers is based in this. They're targeting people who are playing the game, but in doing so are becoming players themselves. One example is Imraan Al-Hariq of V4, who hunts down Ivan Kuznetsov and Tabi Gweneth because they've both killed once. Never mind the fact that Tabi's single kill was [[spoiler:multiple-murderer Clio Gabriella]], and it was only to protect Ivan. What makes it worse though is that Imraan ''acknowledges this and still tries to kill her''.
* In ''SailorNothing'', Himei worries that she is turning into this.
* Satirized by TheOnion, [[http://www.youtube.com/watch?v=EDWo6m5hbG4&feature=player_embedded "Little Boy Heroically Shoots, Mutilates Burglar"]].
* [[Film/VanHelsing Anna Dollerious]] [[http://cleoland.pbworks.com/slod-09-0303-monsters in an installment]] of ''TheSecretLifeOfDolls'', which was named after the trope itself.
* The Union series. Combined Forces - Team 4 falls under this trope. While they start off as idealistic as soldiers go, they eventually devolve into bitter reflections of themselves that kill because they can, not because they have to. Taken to the extreme with Shadow Agents, clones born and bred to go as far into this trope as possible, becoming little better than what they're fighting.
* MovieBob made a reference to this trope in his review of ''Shrek 4'' while he was describing the progression of the ''WesternAnimation/{{Shrek}}'' movies, pointing out that while the first movie was an original and entertaining parody of corporate-driven animated movies, the series had slowly progressed into becoming the very same type of corporate-driven animated movie that the first film had made fun of.
%%%* EquestriaChronicles.
* {{Discussed|Trope}} by Gordon Freeman in ''FreemansMind'', although he can't remember whether Nietzsche said "monsters" or "drummers". He concludes that it must have been "drummers" because there was no way he could end up being anything like the monsters he was fighting, especially not the fifty-foot tall Tentacle he was dealing with at the time.
* ProtectorsOfThePlotContinuum Agents can become rather Sue-ish in their efforts to rid the Multiverse of sues. Of course, they never become anywhere near as bad as those that they are tasked with killing.
** In the mission into ''Fanfic/RainbowFactory'', Caroline goes absolutely postal with the impostor Rainbow Dash, and later says she would probably have gone completely over the edge had Kilroy not hit upon the right word to bring her to her senses.
* Subverted by SFDebris, regarding [[Series/StarTrekVoyager Captain Janeway]]:
--> She has stared into the abyss as it has stared into her... and the abyss said "[[EvenEvilHasStandards JESUS!]]"
* In ''Literature/{{Worm}}'', Taylor begins to epitomize this trope as she gets deeper and deeper into the politics of being a [[DifferentlyEmpoweredPerson cape]], eventually becoming a WellIntentionedExtremist who makes it clear that there are no lines that she refuses to cross if it accomplishes her goals.
* The TropeNamer is ''quoted'' in the teaser trailer for the [[TheSlenderManMythos Slender Man]] series ''[[WebVideo/TribeTwelve Tribe Twelve.]]'' And, sure enough, the series shows signs of this. [[spoiler:It's now inevitable (well, probably) as Firebrand, the Collective member that pulled a FaceHeelTurn (with the help of [[WebVideo/EverymanHYBRID HABIT]]) has been revealed to be a [[FutureMeScaresMe future version of]] Noah, the protagonist.]]
[[/folder]]

[[folder:Western Animation]]
* This is apparently the reason for Shego from ''WesternAnimation/KimPossible'''s FaceHeelTurn (at least going by her Lawful Stupid brother's version of events).
* Depth Charge in ''{{Transformers}}: WesternAnimation/BeastWars'' snubbed most of the Maximal cast to continue his hunt for Rampage, who had murdered the colony he was supposed to protect beforehand. The one time he decided ''not'' to put his vendetta before an important mission, he was ambushed by his prey, and chose to finish him off in a suicide attack rather than return to his duty. Being frequently AWOL allowed him to pull off several BigDamnHeroes moments, but that doesn't excuse his not being there fighting in the first place. He also utters this trope's StockPhrase word-for-word: "It's not revenge I'm looking for, it's justice." Depth Charge even went head-to-head with Optimus over it, when he showed no remorse at Cheetor's death and didn't care about what really happened to him. Optimus-fucking-Primal had to threaten him with a lobotomy to get an answer and some respect. Once Depth Charge opened up a bit, he revealed a fondness for Cheetor.
** Let's also throw in the Maximal government; they locked down all information and locations related to the Great War, Big Brother style, they experimented on their own citizens in order to copy a condition in a ''Decepticon's'' spark that rendered it immortal (resulting in the creation of Rampage and the atrocities he committed, above), and their treatment of the Predacons is very much like the treatment of the Allies towards post-World War I Germany. All in the name of keeping the peace and not letting the atrocities of the past come back to haunt Cybertron again.
** [[BigBad Megatron]] himself is this in several continuities, such as most of the comics series, Beast Wars, TransformersAnimated and [[WesternAnimation/TransformersPrime Prime]], usually getting his start fighting against an oppressive and unjust Autobot/Maximal regime, and then going more mad and power-hungry than the guys he rebelled against. In Animated, he's fallen less far than usual, to the point where he would be downright antiheroic if not for his rampaging [[FantasticRacism anti-organic bias]] and somewhat psychopathic minions.
* General Wade Eiling in ''JusticeLeagueUnlimited'' became so obsessed with taking down the show's titular League (and metahumans in general), that he injected himself with a Nazi-created SuperSoldier serum so he could more readily combat them. His only claim to fame after this? Beating up seven non-powered second-stringers and causing the same superhuman fear-mongering from the citizens that he swore to protect from the League. He did put a LampshadeHanging on this trope at the end.
* Hama of ''WesternAnimation/AvatarTheLastAirbender''.
** Ditto [[WellIntentionedExtremist Jet]], for that matter, although he calmed down a little eventually. And by "eventually", we mean "[[HeelFaceDoorSlam too late]]".
** Also, Fong the Earth Kingdom general, who was obsessed with forcing Aang into the Avatar State.
** Katara in "The Southern Raiders", though she manages to restrain herself at the last minute.
** This is also an element in the propaganda history the Fire Nation feeds its kids.
** In the SequelSeries, ''WesternAnimation/TheLegendOfKorra'' Amon [[spoiler:was the son of Yakone and a young waterbender who was taught by his father to bloodbend. Eventually his father's abuse of both him and his brother Tarrlok, brought him the belief that bending was the root of all evil and needed to be ended.]] By the time the series comes around and by the end of it he has become just as cruel as any bender was.
*** [[spoiler:Hiroshi Sato]] also fits, wanting to eradicate bending to avenge the murder of his wife. However, his thirst for vengeance goes to the point where he loses all capacity for love and [[OffingTheOffspring even attempts to murder his daughter who opposes him and his plans.]] [[CallingTheOldManOut She even tells him that his wife would hate what he's become.]]
* On ''ReBoot'', little Enzo Matrix never took the presence of LawfulEvil Megabyte ''too'' seriously, until Megabyte finally took over Mainframe, exiled TheHero, Bob, into the WWW and pushed Enzo into despair until he was trapped in games. When Enzo came back ''much'' older and ''much'' more serious, killing Megabyte was all he could think of. There was even an episode where, in a simulation of Mainframe, he was in Megabyte's body and easily acted the part unconsciously. All this anger towards a virus that once [[spoiler: [[AffablyEvil gave Enzo a guitar for his birthday]].]]
** However, giving Enzo a guitar was more a 'RuleOfFunny' moment as well as pressure to keep it 'kid friendly' from the meddling executives. Once free from said executives, they were free to make a much darker and less kid friendly story where Megabyte showed his TrueColors.
*** However, [[http://youtu.be/BzSNrwNpw0g when given the opportunity]], Matrix decides that Megabyte is NotWorthKilling.
* A major theme in the "City of Thieves" episode of ''WesternAnimation/AdventureTime'', though it is used more for the RuleOfFunny.
* In ''WesternAnimation/{{Gargoyles}}'', Demona and, later on, Jon Canmore/Castaway. Demona responds to human racism and genocide against her species by gradually turning into a raging, murderous, genocidal maniac who wants to kill all humans. Jon Canmore, to avoid having to admit that he spent his life hunting innocent people and that his own error paralyzed his brother for life, also becomes a genocidal maniac who wants to kill all gargoyles.
* ''AaahhRealMonsters'': Simon the Monster Hunter wanted to prove that monsters were real, and fancied himself the OnlySaneMan, despite how the world at large viewed him as a lunatic. As the series progressed, he developed a personal vendetta against Ickis that drove him to increasingly nastier and ever-more-insane plans, eventually becoming dead-set on getting his revenge on monsters in general and the Trio in particular.
* ''WesternAnimation/SouthPark'' loves this trope. In one particular episode, in order to keep [[ItMakesSenseInContext tomato Kenny]] alive, Stan and Kyle enlisted the Republicans, and showed Kenny on national TV like that...only to learn that his last wish, if he were ever hooked up to a feeding tube, was "...for the love of God, don't show me on national TV like that."
** Kyle gets this most, considering his personal hatred for Cartman (not that Cartman's actions make it any less justifiable). Episodes such as "Fatbeard" display his borderline murderous vendetta against him.
*** In earlier season it was implied that [[SpoiledBrat Cartman]] became such a JerkAss because he was constantly bullied for his weight. "Cartman's Silly Hate Crime 2000" goes so far to imply that Clyde almost underwent the same evolution after the boys turned their attention onto him instead, even taking on some of Cartman's obnoxious attributes.
** The episode "Butterballs" depicts anti-bullying advocates as being [[NotSoDifferent no better that those they are fighting against]]. Or the people that eventually advocate against them...and those that advocate against ''them''...
* ''WesternAnimation/TheSimpsons'' played this trope surprisingly straight in the "Revenge Is a Dish Best Served Three Times" segment "Revenge of the Geeks", in which, the Elementary School's group of nerds, tired of being bullied, created a weapon and gave it to Milhouse, since he's the only one with hand-eye coordination. At first, he uses the invention to get back at the bullies, but then he started to use it on anybody who wronged him in the past, to the point of becoming a bully himself. Lisa, who was narrating the story, says that the aesop of the story is that revenge can make you as bad as the people who harm you. Lampshaded in "Sex, Pies and Idiot Scrapes," where Homer and Ned become bounty hunters and Homer scolds Ned for "not becoming as bad as the people they were hunting."
* After time travelling to a BadFuture, Gosalyn finds that her absence made WesternAnimation/DarkwingDuck turn into Darkwarrior Duck whose idea of justice is DisproportionateRetribution. He is, without a doubt, far worse than any of the villains who he has already disposed of permanently.
* The seeming moral of the animated short "[[http://www.youtube.com/watch?v=jJ-n72sADsg Who's Afraid of Mr. Greedy]]"
* This whole trope was [[AnAesop the moral]] of the ''WesternAnimation/MyLittlePonyFriendshipIsMagic'' episode "One Bad Apple."
** [[LampshadeHanging Lampshaded]] [[FunnyMoments hilariously]] by [[MetaGuy Sweetie Belle]].
--> '''Sweetie Belle''': Why does life have to be so ''ironic?!''
* ''ThePowerpuffGirls'' become this in "Candy Is Dandy." They strike a deal with Mojo Jojo to get an endless supply of candy, but when the candy gives out, they beat Mojo to a pulp like they never had before. It's Blossom [[MyGodWhatHaveIDone who finally realizes this]], the girls becoming as bad as the villains they fight over a candy addiction.
* Korra from ''WesternAnimation/TheLegendOfKorra'' came within spitting distance of this trope during season 2. After her father was arrested on charges of conspiring to kill Unalaq and sentenced to life in prison, Korra arbitrarily decided the judge was corrupt and chased him down after the trial, threatening to have Naga ''crush his skull'' if he didn't tell her what she wanted to know.
* Any rare ''TomAndJerry'' short that Jerry lost to Tom, this trope was often his undoing, being so set in his vendetta against Tom he'd attack him beyond provocation or even draw the first blow.
* ''WesternAnimation/RobotChicken'' does an extrapolation of ''RevengeOfTheNerds'' and points out how what the Nerds do is worse that what was done to them, and that what they did was in fact illegal.
[[/folder]]

%% No Real Life Examples Please. That would be just be asking for an EditWar.
----
->''Don't like it so much, now that the boot's on the other foot?''
----