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[[quoteright:300:[[VideoGame/FableII http://static.tvtropes.org/pmwiki/pub/images/Fable_2_Cover_Art_No_text_5914.jpg]]]]

->''"[[TropeNamer 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.

Our avenger may have [[WellIntentionedExtremist good intentions]]—the fiend may well be too dangerous to be kept alive—but ultimately, his [[AmbitionIsEvil obsession]] with dealing 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; he feels that TeamSpirit is just a hindrance, and that LoveIsAWeakness that he can't afford to have. [[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 Anti-Heroes]] and [[WellIntentionedExtremist Well-Intentioned Extremists]] live just near the boundary, especially the more pitiless GoodIsNotNice, [[TheUnfettered Unfettered]], PragmaticHero types. Engaging in VanHelsingHateCrimes is a good indicator of having ''crossed'' the boundary and a KnightTemplar is most certainly beyond it.

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}} forgo rationality.

May be expressed through the page quote, or via the common misquote "When you become obsessed with the enemy, you ''become'' the enemy".

Not to be confused with CompleteMonster, although somebody can well become one by fighting such monsters. They can range from {{Anti Hero}} 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 TheHorseshoeEffect, 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 HunterOfMonsters. Subtrope of SlowlySlippingIntoEvil.

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

* HeWhoFightsMonsters/AnimeAndManga
* HeWhoFightsMonsters/ComicBooks
* HeWhoFightsMonsters/FanWorks
* HeWhoFightsMonsters/{{Literature}}
* HeWhoFightsMonsters/LiveActionFilms
* HeWhoFightsMonsters/LiveActionTV
* HeWhoFightsMonsters/TabletopGames
* HeWhoFightsMonsters/VideoGames

%%% 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.


[[folder:Films -- Animation]]
* ''WesternAnimation/BatmanMaskOfThePhantasm'': Alfred also refers to the Nietzschian quote, in regards to [[spoiler:Andrea aka the Phantasm]].
-->'''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. [[spoiler:But Andrea fell into that pit years ago, and no one, not even you, could have pulled her back.]]
* ''WesternAnimation/BatmanUnderTheRedHood'', like ''Phantasm'', examines this Trope, only this time in regards to [[spoiler:Jason Todd]].
* 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 ''WesternAnimation/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'', after Batman's OmnicidalManiac AlternateUniverse counterpart Owlman gives him a NotSoDifferent speech, Batman's eventual rebuttal (slash PreMortemOneLiner) references the Nietzsche quote at the top of the page.
-->'''Batman:''' There ''is'' a difference between you and me. We both looked into the abyss, but when it looked back at us... ''you blinked''.
* ''WesternAnimation/SupermanVsTheElite'': Superman shows [[BewareTheSuperman how frightening]] he can be to people who [[BeCarefulWhatYouWishFor wished he would cross this line]].
* In ''Disney/BigHero6'' Hiro [[spoiler:attempts to use Baymax to kill Yokai/Robert Callaghan. However, both his team and Baymax help him [[DefiedTrope defy]] this trope when they bring him back to his senses.]]
** This is what happened to [[spoiler:Yokai/Robert Callaghan]] himself - [[spoiler:his daughter's apparent death]] drove him to extremely desperate measures to [[spoiler:take revenge on Alistair Krei]].
* ''WesternAnimation/ParaNorman'': The [[spoiler:so-called]] WickedWitch [[spoiler:Agatha Prenderghast]] was so intent on punishing those who wronged her that she became just as angry and spiteful as them. Norman calls her out for wanting to hurt people just because [[spoiler:she's scared and angry, just like the townspeople did to her]].
* The title character of ''Anime/RingingBell'', motivated by the senseless killing of his mother by the Wolf King, goes on to become just as ruthless and bloodthirsty as the wolf.

* Some tropes come about as a deliberate attempt to defy a stereotype or cliché, only to become as over-exposed as the very thing they were defying. A good example is RealWomenDontWearDresses, which began life as an attempt to defy the stereotype of women as being weak and overly-sensitive, only to become a stereotype in and of itself that suggested that women who don't wear masculine clothing or act in a stereotypically masculine way are inferior to those that do.

* 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\\

* 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]]''


* In Myth/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 ''VideoGame/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.
** ''Saiyuki'''s portrayal of this myth especially invokes this: while the various anime adaptations more or less consistently show that merely killing a thousand youkai turns one into a youkai, in the manga Hakkai's case is unique, and WordOfGod specifically states that it's not the act of killing or "being bathed in youkai blood" that triggers the transformation, but the killer's inner negative energy. In other words, this can only happen to those who already lost enough of their humanity and cultivated enough of their hatred that they might as well be compared to monsters themselves.

[[folder:Professional Wrestling]]
* Wrestling/StoneColdSteveAustin's Heel Face turn during the Wrestling/{{WWE}} Attitude Era. After years of battling Vince [=McMahon=] and eventually Wrestling/TripleH who was with Vince, Austin out of desperation to become the WWE Champion again, does a WeCanRuleTogether and reveals he joined Vince at Wrestlmania XVII. Cemented during the following Monday Night Raw broadcast where Steve Austin publicly beats up his best friend Wrestling/JimRoss who openly spoke against Austin's decision.
** Austin was on the other side of it a few years earlier during his feud with Wrestling/BretHart. Austin kept going after Bret, calling him out, and attacking him at every opportunity. Getting beaten by Bret in matches didn't discourage Austin, who kept hounding him. So it really shouldn't have been a surprise that Bret finally got angry enough to absolutely brutalize Austin in their famous submission match at Wrestling/WrestleMania, which was the beginning of his FaceHeelTurn.
* [[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.
* Hinted at the end of Wrestling/KevinSteen and [[Wrestling/SamiZayn El Generico]]'s confrontation at Wrestling/RingOfHonor Final Battle 2010. Generico, after suffering a whole year of his former friend's abuse, has Steen on his knees and is holding the very chair that was used to start the feud a year earlier. As Generico swings it, Steen holds up Generico's former mask, as if to suggest actually hitting him would make him just as bad as he is. Generico holds the mask up to his face, contemplates it... [[spoiler:[[AvertedTrope And realizes that no, Kevin Steen is just a complete scumbag who deserves to get hit in the face.]] He drops the mask, smacks Steen with the chair, and pins him for the victory.]]
* Wrestling/{{Carl|itoColon}}y originally had no interest in becoming a pro wrestler and was happy to just help around the family business, working as a camera man in WWC. This changed when Ray González began targeting Carly because he was feuding with Carly's father at the time. 14 years later, Carlito Caribbean Cool is one of the top wrestlers in WWC and is targeting Ray González Jr, who till then was content to simply help with his father's mini market.

%%%* Aeschylus's ''[[Theatre/TheOresteia Oresteia]]'' trilogy.
%%%* ''Theatre/{{Hamlet}}'' is the most well-known play in the Revenge Tragedy genre.
* OlderThanSteam: ''Theatre/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...]]
* Theatre/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:Literature/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:Visual Novels]]
* ''Franchise/AceAttorney'':
** [[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.
** [[spoiler:Phoenix Wright]] himself becomes this in ''VisualNovel/ApolloJusticeAceAttorney''. After having his attorney's badge taken away for [[spoiler:(accidentally) presenting forged evidence, he loses respect for the whole judicial process, and manipulates crime scenes, forging evidence to get Kristoph Gavin convicted.]] He gets better by Dual Destinies.
** [[spoiler:Damon Gant]] used increasingly unethical methods ([[spoiler:including murdering a subordinate and framing a 15-year-old girl for it for the sole purpose of blackmailing her sister into framing someone else]]) to increase his control over the investigative process and neutralize criminals who could not be convicted otherwise, such as Joe Darke.
** In the sequel to ''VisualNovel/AceAttorneyInvestigations'', the final villain ([[spoiler:Simon Keyes]]) was driven to commit a large array of crimes including [[spoiler:kidnapping two minors and framing both of them for murder, killing a man with a falling hot-air balloon and arranging the murders of two other people, all so that the murderers would be caught and arrested]] because [[spoiler:he had been a witness to a murder 12 years earlier but was silenced by the joint conspiracy of Blaise Debeste, Patricia Roland and the fake Di-Jung Huang and forced to go on the run in order to avoid being killed. Edgeworth even points out that his motive was a mix of justice and revenge but that he had become as bad as his enemies.]] At the end of the game, Edgeworth pledges to capture corrupt people in power before their victims become examples of this trope.
* Archer from ''VisualNovel/FateStayNight''. The famous "Unlimited Blade Works" poem that he had supposedly written about himself even carries a message that is very similar in nature to the "He who fights monsters" quote itself (if you manage to decipher its incomprehensible language that is). It tells the story of a man who spent his whole life in an agonizing pursuit through self-sacrifice, to become the hero of his ideals, until he one day realized that it was an ideal that could never be met, that evil would always persist, that all his sacrifices had been in vain, and that he had ultimately only managed to turn himself into a killer without a cause. He was just another part of the evil in the world that he once had thought he was fighting. The man who wielded swords had thus lost his identity to the point where there no longer was a man, there were only his weapons. He was "a man made of swords".

[[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.
* ''WebComic/ParanormalMysterySquad'': Stephanie is introduced as having a staunch [[VanHelsingHateCrimes zero-tolerance policy]] against cryptids, because of what happened to her parents. But by the time of the ''[[WebComic/VampireCheerleaders "Vampire Cheerleaders Must Die!"]]'' arc, she's [[CharacterDevelopment come to accept that]] [[http://www.vampirecheerleaders.net/strips-vc/my_dark_passenger not all cryptids are evil as she first thought]], as a result of her sister becoming a werewolf and after spending time with their friends, who're also cryptids. [[spoiler:Stephanie later becomes a rare literal example and figurative subversion when she becomes a cryptid herself.]]
* ''Webcomic/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 ''Webcomic/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.
%%%* ''Webcomic/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.
%%%* As is usual, ''Webcomic/PennyArcade'' gives us quite the [[http://www.penny-arcade.com/comic/2008/4/11/ atypical example.]]
* Professor Broadshoulders from ''Webcomic/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.
* In ''Webcomic/GirlGenius'', the main reason that the BigBad the Other, aka Lucrezia Mongfish, is so reviled is her use of [[MindRape mind-control via slaver wasp]]—if one [[NightmareFuel flies into your mouth]], you become susceptible to any order given by her—one character is ordered to die, [[http://www.girlgeniusonline.com/comic.php?date=20061113#.Vvqv8OIrLIU and does so immediately]]. [[WellIntentionedExtremist Baron Wulfenbach]] is her main enemy, with the stated aim of destroying her and all her work, no matter what it takes. Which makes his [[GrandTheftMe personality overlay]] onto [[spoiler:his son, Gil]] extremely hypocritical, especially since this is ''[[NotSoDifferent the exact same thing]]'' that Lucrezia did to [[spoiler:her daughter Agatha]]. It's called out by the BigBad herself in comic, that in trying to protect [[spoiler:Gil]] from her influence he's ended up becoming just as bad as she is.
--> '''Lucrezia''' '''[[spoiler:in Agatha]]''' ''':''' My goodness! How Mister High-and-Holy has fallen! Using your '''''[[spoiler:own son?]]'''''

[[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.
* FightingMonsters is named after this trope, and plays with it thoroughly, bringing up the original quote in the prologue, as well as throwing it at the main character in the form of a WhamLine.
* The Dove, from the ''Roleplay/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 ''Roleplay/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 ''Literature/SailorNothing'', Himei worries that she is turning into this.
* Satirized by Website/TheOnion, [[http://www.youtube.com/watch?v=EDWo6m5hbG4&feature=player_embedded "Little Boy Heroically Shoots, Mutilates Burglar"]].
** '''[[BloodKnight I want the blood.]]'''
* 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.
* [[Creator/BobChipman 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.
%%%* Roleplay/EquestriaChronicles.
* ''WebAnimation/{{RWBY}}'': The White Fang was once a peaceful protest group, trying to advocate peace between humans and [[LittleBitBeastly Faunus]], but five years prior to the start of the series, a faction within the group, feeling that said peaceful protests were getting them nowhere, staged a coup and took control of the entire organization, twisting it into a violent terrorist group who, by the time of the show's beginning, has become no better than the humans who persecuted and shunned them; even other Faunus find the White Fang's violent actions actions sickening.
* {{Discussed|Trope}} by Gordon Freeman in ''Machinima/FreemansMind'', although he can't remember whether Nietzsche said "monsters" or "drummers". He concludes that it must have been "drummers" [[ComicallyMissingThePoint because there was no way he could end up looking anything like the monsters he was fighting, especially not the fifty-foot tall Tentacle he was dealing with at the time.]]
* WebOriginal/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 WebSite/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 [[DifferentlyPoweredIndividual 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 [[Franchise/TheSlenderManMythos Slender Man]] series ''WebVideo/TribeTwelve''. 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.]]
* Starting to happen to Percy in ''WebVideo/CriticalRole''. He starts as the OnlySaneMan in the party, but when he finally encounters the people who slaughtered his family, he snaps, and starts to [[ColdBloodedTorture torture prisoners]] amongst other things.
* ''Series/CobraKai'' gets hit ''hard'' with this trope, when Johnny sees the results of his dojo at the All Valley Tournament. Miguel, Aisha, and Hawk just wanted to be able to defend themselves against the students bullying them. While Johnny's training helped them achieve those results, it also led to them becoming the similarly ruthless, aggressive brutes that his own gang had once been. And he's like, OhCrap

[[folder:Western Animation]]
* This is apparently the reason for Shego from ''WesternAnimation/KimPossible'''s FaceHeelTurn (at least going by her LawfulStupid brother's version of events). More likely, she was fed up with [[DefectorFromDecadence their stupidity]].
* Depth Charge in ''Franchise/{{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, WesternAnimation/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 ''WesternAnimation/JusticeLeague Unlimited'' 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.
-->'''General Eiling:''' ''[about to throw a car onto Shining Knight]'' Superman and your Justice League are a threat to a safe and stable world.\\
'''Elderly Woman:''' ''[standing between Shining Knight and Eiling]'' Drop the act. You think killing Superman would make the world safe? Or killing this boy? Or us? [[ArmorPiercingQuestion Tell me, how many of us do you have to kill to keep us safe?]]\\
'''General Eiling:''' They're the ones I'm after, not you. I'm not the menace, metahumans are! Superpowered beings!\\
'''Stander-by:''' [[ArmorPiercingResponse You're the only one around here with superpowers!]]\\
''[Eiling stops, dropping the car]''\\
'''General Eiling:''' ...All right, [[YouAreWhatYouHate I've become what I hate,]] [[LampshadeHanging I'll give you that.]] But in the long run, you'll see I was right. You'll see you need the likes of me to protect you from them.
** And before that, the Justice Lords, the alternate reality counterparts to the league who fell to this after Lex Luthor became president and killed the Flash.
* ''WesternAnimation/AvatarTheLastAirbender'' features a lot of these.
** [[WellIntentionedExtremist Jet]] was first established as a freedom fighter similar to Robin Hood, but was later shown to be completely willing to take some innocent lives in order to kill some Fire Nation soldiers. He calmed down a little eventually. And by "eventually", we mean "[[HeelFaceDoorSlam too late]]".
** Fong the Earth Kingdom general, was obsessed with triggering the highly destructive Avatar State in Aang by ordering his men to attack him. He even went as far as to endanger Katara's life, by burying her alive.
** Hama was an imprisoned waterbender who taught herself the dark art of bloodbending, allowing her to control the bodies of her victims in a rather painful way. She used this to escape the Fire Nation prison. In her later life she became at least as destructive and cruel as the fire benders she hated, as she used her bloodbending to abduct innocent Fire Nation citizens.
** Katara herself almost becomes this trope in "The Southern Raiders". In order to find closure to the death of her mother, she decides to hunt down the man responsible for it, going as far as to willingly use bloodbending, a technique she really did not want to learn in the first place. When she does find her mother's killer, she almost impales him with a barrage of icicles, but manages to restrain herself at the last minute.
* This is a recurring theme in the SequelSeries, ''WesternAnimation/TheLegendOfKorra'', where each of the main antagonists stands for an ideal, but takes said ideal too far, resulting in them becoming the villain.
** Amon [[spoiler:was the son of the mob boss 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.]]
** Korra herself 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 ''bite his head off'' if he didn't tell her what she wanted to know.
** Zaheer wanted freedom for everyone and saw all the government corruption, incompetence and tyranny as something that needed to be dismantled completely in order to provide that. While no one shed tears over his first victim, the tyrannical Earth Queen, he still began an uprising and had plans to assassinate every world leader to plunge the world into complete anarchy. While he disdained hurting innocents, he wouldn't hesitate to do it to further his goals.
** Kuvira was the result of Zaheer's uprising. Originally a brave and heroic woman, she desired the stabilization, safety and reuniting of the Earth Kingdom after Zaheer's uprising. The stabilization took three years to accomplish and she was determined to never let it happen again, nor let the circumstances that led to Zaheer to come back. So, she overthrew the outdated monarchy and established the Earth Empire, becoming an even worse tyrant than the previous Earth Queen ([[WellIntentionedExtremist albeit with much more noble intentions]]). [[spoiler:Korra revealing to her just how far she had fallen is what finally convinced her to surrender]].
* On ''WesternAnimation/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.
* ''WesternAnimation/{{Ben 10 Ultimate Alien}}'': Charmcaster becomes this [[spoiler: after her best efforts in resurrecting her father drove her to madness and insanity to the point where she became even worse than the previous tyrant who killed him in the first place. After her father calls her out on this, [[HeelFaceTurn she gets better]]...[[HeelFaceRevolvingDoor mostly]]]].
* 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.
* ''WesternAnimation/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. (Homer, however, [[AlternateAesopInterpretation interpreted it as "Never put down your weapon."]])
** 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." Whether or not it was done so successfully is a point of contention in the fandom.
** [[LampshadeHanging Lampshaded]] [[SugarWiki/FunnyMoments hilariously]] by [[MetaGuy Sweetie Belle]].
--> '''Sweetie Belle''': Why does life have to be so ''ironic?!''
* ''WesternAnimation/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.
* Any rare ''WesternAnimation/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 ''Film/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.
* ''WesternAnimation/SpongeBobSquarePants'':
** In the pre-movie seasons, Mr. Krabs was originally the BigGood to Plankton's BigBad, simply defending his business against Plankton's {{Evil Plan}}s. Post-movie, due to {{Flanderization}}, he's often shown to be just as evil and cruel as Plankton, if not worse, and often makes it his personal duty to sabotage Plankton when he's making legitimate profits or simply isn't miserable. In "The Krabby Kronicle," ''Plankton himself'' is actually disgusted at how low Krabs would stoop to make money:
-->'''Plankton''': And I thought ''I'' was evil!
** Compared to Squidward, who openly hates [=SpongeBob=] and doesn't bother hiding it, Mrs. Puff was a docile, well meaning character whose nerves have slowly been worn at by Spongebob's antics. Several episodes have shown her take increasingly desperate and sometimes unscrupulous measures just to finally get away from him and in one episode, she outright tried to ''murder'' him.
* Bismuth from ''WesternAnimation/StevenUniverse'' is this due to her making a weapon designed to kill her enemies (Homeworld Gems) with has plans to commit genocide against them and will destroy anyone who stands in her way (even allies), thus becoming as bad as the Homeworld Gems she fights against who have done the same against their enemies and allies.
* Frequently lampshaded by Kion in ''WesternAnimation/TheLionGuard''. He was bestowed with the Roar of the Elders, which is given to the second-born cub, as the first is destined to be the king, while the 2nd is tasked with leading the Lion Guard, a group dedicated to preserving the kingdom and defending the Circle of Life. As its leader, the second-born cub is expected to be the fiercest of the group and is thus gifted with the Roar of the Elders by the Great Kings of the Past. Kion knows that Scar, Simba's grand-uncle, was bestowed with the Roar when Mufasa was heir, but used it for evil purposes, causing the Great Kings to strip Scar of the roar. Therefore, Kion's greatest fear is to become NotSoDifferent from Scar, and it is this reason that forces him to regulate his use of the Roar.

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