In Mahou Sensei Negima!, Haruna identifies Kurt Godel as being 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 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. 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 prevents her from going down his brutal way.
Scar of Fullmetal Alchemist. 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 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 tracks down Envy and burns him nearly to death. It takes Ed, Scar, and Hawkeye to talk him down from dealing the final blow in a fit of vengeance. And then Envy, seeing how strong humanity can be, 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 Father Anderson's decision to turn himself into a literal monster in order to remove Alucard from the world. He fails, but only closely.
Afro Samurai. Reread the first paragraph. That describes, in a nutshell, the entire first season of Afro Samurai.
In the Meakashi chapter and by extension the Watanagashi chapter of Higurashi No Naku Koro Ni, Shion Sonozaki becomes convinced that her family kidnapped and/or murdered her Love Interest, Satoshi Honjou, and starts on a chain of murders of her own to avenge him, culminating in murdering his little sister when his very last request had been for Shion to protect her. This leads to a Heel Realization and Ignored Epiphany.
The monster hunters of Claymore use 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, when they over-exceed their limit. However, there was one particular time when Clare was willing to give up her humanity in order to kill Priscilla when she encountered her at last.
Code Geass's Lelouch is fully aware of and, in fact, embraces this trope.
Naruto is centered a lot around revenge, mainly on Sasuke's side. He wants to fight his supposedly evil brother and therefore joins the evil side. 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 realized that he was manipulated by Obito after he listened to the First Hokage's story about Madara and his dreams for the village. To make matters worse, this is complete with Karin beingOk 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 kill the Uchiha Clan after, at least according to 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. 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.
Hanzo of the Rain village is also implied to have fallen to this trope; in his youth, he was a Worthy Opponent 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 Moral Event Horizon when he attacks Nagato and Yahiko's peaceful protest group out of fear that they'd try to usurp him. This results in Yahiko and dozens of others dying horribly, Konan being scarred for life into an emotionless Broken Bird, and Nagato being crippled for life, going insane, and becoming the aforementioned Pein.
Naruto himself almost fell into this trope; Early Post-Time Skip, his fury at the douchebaggery of Orochimaru and Akatsuki causes him to go 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 Yu Yu Hakusho, 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 his guilt over his failure to protect his disciples, and he wishes for an opponent strong enough to defeat him.
In Last Exile, Alex Rowe skates dangerously close to this. Although he never makes the jump into full-out villain territory, his need for revenge again Maestro Delphine and the Guild is all-consuming, and at one point, he 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'.
In 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 even 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 his need to protect Casca from the monsters he fights, and, occasionally, 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 worse.
As if that weren't enough, due to his unstable personality, which developed in the face of unreasonable adversity, coupled with the effects of this trope, Guts now struggles with a rather powerful 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 Gun X Sword. At least the protagonist Van, while skirting this line, still made standards here and there about not ruining properties, taking innocents's lives, etc. Ray, on the other hand, will gladly kill any innocents if they stand between his way and The Claw. He even claims that, just as long as he can kill The Claw, he doesn't mind becoming a sadistic murderer himself.
Subverted when he wastes his only chance of killing his nemesis by purposedly missing his last shot at him in order to foil his plan once and for all.
Howl, from Howls Moving Castle (the movie, not the book), is 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.
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.
This trope is given a bit of a workout in Monster: 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 Shakugan no Shana, 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 restricting his harvesting to protect the balance among Friendly Neighborhood Vampires and humans, but is also under Shana's protection, thus resulting in fighting someone who should be her ally.
Char Aznable of Mobile Suit Gundam 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 Char's Counterattack.
In Gundam 00, Louise Halevy becomes this as she turns into a Dark Action Girl 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 Despair Event Horizon to stop her from hurting herself any more.
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 Crowning Music of Awesome 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 Gundam Age, heartbreakingly.Flit has been a 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 Human All Along, as Flit refuses to recognize them as fellow humans who are also suffering in the hands of the evil Earth Federation.
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 Moral Event Horizon. 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 Chirin No Suzu, motivated by the senseless killing of his mother by the Wolf King, goes on to become just as ruthless and bloodthirsty as the wolf.
Bleach gives us Kaname Tousen, who joins the Big Bad because he wants to seek "justice." Turns out all he really wanted was revenge.
In The Tower of Druaga, whoever reaches the top of the tower, defeats the monster Druaga, and gains the 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. Fallen Hero Gilgamesh held out for a century before making his Face Heel Turn, and then, he retained his humanity. Neeba, however, made his Face Heel Turn BEFORE he got the rod and wasted no time going One-Winged Angel.
The El Baile De La Muerte arc of Black Lagoon is an exploration of this trope in regards to Roberta, who sets out on a Roaring Rampage of Revenge against the people responsible for the kaboom that killed her master, a rampage that has her going Ax Crazy during the course of it. 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 Yu-Gi-Oh! GX, 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 It's All My Fault. 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.
Even after his semi-Heel Face Turn, Accelerator in To Aru Majutsu no Index will mercilessly kill anyone who harms innocents, or even worse, try to harm 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 who fits "the hero" type.
In his first appearance in Kirby: Right Back at Ya!, 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 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 Ghost in the Shell. 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 Sayaka in Puella Magi Madoka Magica, though part of it is due to her despair at slowly 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, even before she turns into one herself.
This is the fate of every Puella Magi. If their Soul Jar ever becomes fully 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.
Muteki Kanban Musume: When Megumi accuses Miki (a prototype of The Bully 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 Malicious Slander, sabotaging Miki’s work and setting tramps against Miki – a realistic depiction of The Bully in Real Life.
In Fairy Tail, it's been stated that those who use too much dragon slayer magic take the risk of becoming a dragon themselves.
Yuu from Holyland fears that his taking the fight to delinquents and gangsters as a Bully Hunter makes him little better. Various characters try to assure him that it doesn't.
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 (depending on who's writing him).
It's also why Batman so strictly adheres to Thou Shalt Not Kill: having that as a line that henevercrosses is a 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.
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 Gotham Girls- 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 a fact cemented by Dora's own Karmic Transformation 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.
The Punisher from Marvel Comics is often presented this way whenever he makes a guest appearance in more idealistic books like Spider-Man or 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 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...
Rorschach from Watchmen describes this in great detail, recounting how he became a dark and gritty Anti-Hero first (though he had a violent childhood) by brutally taking out his anger and disgust on a kidnapper who had butchered and fed a little girl to his dogs by 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 — 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, where the freighter collects his condemned soul.
Ozymandias could count toward this as well, seeing as 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 Incredible Hulk. 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: a Hulk. He even lampshades it.
Spider-Man 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.
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 Red Lanterns, especially Laira, who was the first Green Lantern 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 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 DC Universe 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. 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 The Walking Dead 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 IDW comic books was this. He initially formed the Decepticons to try and bring 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.
While Superman rarely comes close to this trope, he is very aware of it and lampshades it on several occasions, mostly in response to Anti Heroes 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.
Reed Richards of the Fantastic Four blatantly averts this trope when dealing with his Arch-Enemy, Doctor Doom. 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 Kingdom Come, 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 New Atheist Militia is inspired by Kipling's example and sets about killing gods - by committing genocide 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 Red Hood And The Outlaws.
In Aeon Natum Engel, 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).
A central theme in Avatar:The Last Airbender Revised 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 Arc Words for the first three books to reflect this.
Not only the theme but the name of He Who Fights Monsters, a Rosario + Vampire 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 Improvised Weapons, and killed 5 students, including Inner Moka. "When your life's on the line, there's nothing that you won't do to survive."
Mean Girls 2 has this. Tyler points out in one scene that Jo is acting a lot like Mandi.
James Camerons The Abyss is a literal and figurative interpretation of this. The movie opens with part of the Neitszche quote*
the home video version did, not the theatrical release, thanks to another movie using it that year
, 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.
All the King's Men in a nutshell: Willie Stark counters corrupt politicians but becomes one in the process.
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.
Two-Face: You either die a hero, or live long enough to see yourself become the villain.
Batman Forever 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."
Kevin Bacon's character from the 2007 film Death Sentence. Lampshaded by the gang leader in the ending, who, in a Not So Different moment, revels at the fact that he has reduced the protagonist to such a pathetic shell of a man.
In Fail-Safe, 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 justified 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 Fight Club. 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.
FIST: over the course of the film, the titular labor union begins to act more and more like the Corrupt Corporate Executives 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 Halloween 5 The Revenge Of Michael Myers, 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.
Inglourious Basterds had the eponymous Basterds treating the Nazis in the same manner that they do the Jewish people.
Batman: There is a difference between you and me. We both looked into the abyss, but when it looked back at us... you blinked.
The Anti-Villain from Law Abiding Citizen. 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.
Yuri Orlov in Lord Of War 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 Manhunter and Red Dragon is a criminal profiler who lives in fear that his understanding of the mind of a killer will turn him into a sociopath.
PG-13 teen comedy example: Cady of Mean Girls starts off infiltrating a Girl Posse to get revenge on the Alpha Bitch, 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.
Steven Spielberg's 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 Purge: 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 Ravenous. The villain gained power through cannibalism and the only way the protagonist can fight him is by partaking in cannibalism himself.
In Red Dawn, Tanner, a professional military officer, is concerned about this regarding the kids:
Tanner: "All that hate's going to burn you up, kid."
Robert:(carving 'kill' notches on his AK-47 with a balisong) "Keeps me warm."
Red Hill has this as an important part of the plot. 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 Reign of Fire 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 Rag Tag Bunch Of Misfits in the 1995 remake of the movie Sahara. He was sent by La Résistance to Africa to fight as a regular soldier since he had "too much hate [for the Germans] to make good Resistance."
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 will kill himself when he's done.
Star Trek First Contact: 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.
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.
And in Return Of The Jedi, 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.
Anakin, before he became Vader. Let us not forget he not only slew the Tusken Raiders responsible for Shmi's death, he also "killed the women and the children."
Lampshaded by Obi-Wan Kenobi in Revenge Of The Sith: Right before his dual with Anakin, he points out that Anakin has become the very thing he sought to destroy, a Sith Lord.
The eponymous character of Sweeney Todd: The Demon Barber of Fleet Street, 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, 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 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 seriousBreak the Cutie ordeal to boot. We've got a serious Downer Ending here.
Sarah Connor in Terminator 2. Her personal mission was to stop Judgment Day from occurring and keep Skynet from forming. To accomplish this, she hid from the law, raised her only child to become a military leader, shacked up with gun runners, 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.
Thirteen Days. 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 Sympathy For Mr Vengeance, 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 Oldboy. Both Oh Dae-su and his target Lee Woo-jin are 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 Sympathy For Lady Vengeance, 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.
Magneto in X-Men: First Class notes that he and Shaw are exactly the same and admits that he would work with him...except that Shaw killed his mother. And that's the only reason he's going to kill him, not to save humanity or stop Shaw's plans.
Dreamworks Animation's Monsters vs. Aliens has an interesting version 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.
This seems to be the overall theme of Star Trek Into Darkness, with the "monsters" being both 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. 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.
As mentioned in film section above, Willie Stark from All the King's Men becomes the kind of politician he once meant to oppose.
Bartemius Crouch, Sr. from Harry Potter, 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.
Robert Neville, protagonist of Richard Matheson's famous vampire novel I Am Legend, is seen this way for killing both the feral vampires and those who have enough sanity to attempt a rebuilding of society. In his defence, he didn't have a way or an incentive to tell the difference.
The original ending of the 2007 movie version had him discover that they are all somewhat intelligent and realize what a monster he's been. He is then able to reconcile with the vampires peacefully and leave. While in the theatrical ending, he doesn't and dies killing them, which, in light of the original ending, fits this trope without actually intending to.
The novel ended with 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 Eisenhorn novels set in the Warhammer 40000 universe chronicle, in first person, the struggle of Inquisitor Eisenhorn against the vile forces of 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 Gaunt's Ghosts 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, Ravenor. Ravenor explicitly stated that the Jump Off The Slippery Slope is inevitable when you spend so long fighting monsters - what matters is how much good you do beforehand.
In Do Androids Dream of Electric Sheep?, the book that inspired the film Blade Runner, Decker meets a fellow bounty hunter, Phil Resch, who hates Replicants a good deal more than he does. He's got a good reason for that, and, considering what the other guys who went through the experience were like, he got lucky.
This trope is a central theme of the book. Deckard often doubts that he is any more human than the 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 Terry Pratchett's non-Discworld novel, Nation. The heroine has been warning the hero about the Big Bad, 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 Big Bad, 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. 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 Yellow Peril 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 thought) and tried to stop an European colonization via giving the locals-to-be-colonized somewhat more advanced weaponry. They succeed... but the local development was a bit faster and 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 Curbstomp Battle.
How it came to this? How we who hated missionaries became missionaries ourselves before we knew! Missionaries of rocket launchers...
In Codex Alera, 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 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 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 he subverts it gloriously at the end of The Gathering Storm.
In the Star Wars Expanded Universe, 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 Luke Skywalker (several times, always ending with a Heel Face Turn), Yuthura Ban, Quinlan Vos, Darth Revan (possibly), Depa Billaba...
A quote attributed to Yoda is a paraphrase of the page quote: "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 Warhammer 40000Grey Knights 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.
The Kingpriest from Dragonlance started out as The Messiah, but as he became increasingly confronted with corruption in the world, his quest to purify it became more and more unhinged. In the end, he was hardly better than the people he was fighting and 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 Captain Kennit in the Liveship Trader trilogy by Robin Hobb. 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 Freudian Excuse in this case.
Robert S. Pierre and Oscar Saint-Just in the Honor Harrington 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 PragmaticVillainy. 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 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 State Sec and for 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 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 His Dark Materials, two of the major nemesises—Lord Asriel and Marisa Coulter—embody Sydney Harris's maxim (quoted above) over the course of the trilogy. 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 Witchers are monsters, angry/scared peasants (as a monster), and other Witchers.
In Jim Butcher's The Dresden Files, the book Fool Moon has 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 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 Hellfire has upon him.
Now that Harry's 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 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 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 Anita Blake 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 realising that she was this all along. At the end of the 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.
In The Hunger Games trilogy, District Thirteen does a bit of this with their rules and war tactics. Katniss also calls Gale out on his tactics and standards that are very similar to that of their enemies. Also happens to Katniss and some of the other Hunger Games victors when they decide to hold another Hunger Games with Capitol children in order to get revenge for the district's suffering.
A major theme of Animorphs. The protagonists often worry about how to defeat their enemies without becoming just like them. And 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 Despair Event Horizon.
Rachel became this. Before her 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.
The Doctor Who New Adventures novels, which featured a Doctor who, in the name of defending the universe from evil, would not only Shoot the Dog but subvert history over a hundred years to make sure the dog and the gun were in the right place, and then blow up the planet just to be on the safe side, often contemplated the Nietzsche quote.
Hercule Poirot knew he possessed both the ability and the ego to become the very kind of serial killer he was always working to put behind bars. It's why he deliberately hastens his own death in "Curtain", to ensure he never gives into this kind of temptation.
Sisterhood series by Fern Michaels: 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 Smug Snake, but the Disproportionate Retribution inflicted on him caused the group to sink to his level! To make things even creepier, the author consistently portraying Revenge as a good thing with no bad effects, and the author consistently trying to make the protagonists seem good and heroic despite their actions saying otherwise, really sabotage the series!
In the New Jedi Order 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 Jump off the Slippery Slope.
It turns out in the final book that this trope was the Yuuzhan Vong's own Start of Darkness.
In Out Of The Dark 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 which turns out to be untrue 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. 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, 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, Silvia nearly falls into ths trap herself, but manages to retain her humanity due to outside help.
Live Action TV
Most of the Argent family from Teen Wolf. 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 Knight Templar, 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 Manipulative Bastard.
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.
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 he 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.
Holtz from Angel. He 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, to the point of kidnapping his son, Connor, and raising him to hate Angel. 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.
On Doctor Who, the Technical Pacifist 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 Sliding Scale of Idealism Versus Cynicism quite frequently. In one case, he was attacked by creatures who wanted to steal his immortality. They got their immortality all right. Getting the Doctor personally angry is, in his own words, "not a good place to stand."
In the Tenth Doctor's final TV appearance, The End of Time, they go into detail about what was going on at the end of the time war. After both sides had done too much messing around with space wedgies and paradoxes, the 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, the Time Lord leadership would have destroyed all of reality because they (and they alone) would be able to survive outside of time as beings of pure energy and information.
In the Doctor Who episode "The Pandorica Opens", the 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 Wham Line for the Doctor is an understatement. He seems more shaken at the idea of being the thing in the Pandorica than at the fact that he's getting locked inside.
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 "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: 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-heroy 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 What the Hell, Hero? 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 even if it means getting dirty, and becoming a Knight Templar. It really puts the times the Tenth Doctor made you ask "isn't he supposed to be the good guy?" in new light.
Happens in the episode of the Twilight Zone, "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 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.
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 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 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 Death Seeker 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 kills Amy Pond (not that one) because he can't trust a monster not to kill again, complete with a 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 demon-blood-fueled powers also lets him save the hosts when exorcising demons, his obsession with gaining the power to kill Lilith leads him to break the final seal, releasing Lucifer from Hell.
Future Dean in "The End" (5x04). After losing his brother to the devil 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.
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 kill defense attorney Ellen Wolf, and 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 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 both Marina and Renzo, the former shooting her brother's murderer and the later burying the main villain of the season... ''alive''.
HRG in Heroes. 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 taking Sylar's power in order to save the world caused him to also gain Sylar's hunger.
One season 9 episode of Law & Order: Special Victims Unit had a serial killer found dead in the same manner as his victims. 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.
This was the origin of the title character in Xena: Warrior Princess. 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.
In season seven, Tony from 24 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.
Jack Bauer takes this trope Up to Eleven in the second half of the eighth season, when he gets his hands on the man who murdered Renee. Unlike Tony however, Jack eventually backs down when he realizes what the consequences (to innocent people) will be if carries out his revenge.
The re-imagined Battlestar Galactica saw the Resistance on New Caprica using suicide bombers against the Cylon occupation force. Colonel Tigh gives us this quote.
"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 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 get even with Kovac, who had treated him.
Dark Shadows has Reverend Trask, a self-styled witch hunter who had undoubtedly killed many innocent women. As a ghost, he's finally talked into a Heel Face Turn thanks to the opportunity to finally destroy a real witch.
LOST: considering who the series's ultimate Big Bad is, this can be inferred as the reason for much of the Others's villainous behavior.
On NCIS, Eli David, Ziva's father, is pretty obviously this. He has crossed the Moral Event Horizon a number of times. 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 Criminal Minds, the actual 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, 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.
The Mentalist: 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. Geez.
Until season 4. Jane, frustrated by his inability to plausibly stop a local serial killer, goads the man into taunting Red John on tv (we all know how that ends). Bonus points for Jane not only turning into what he hunts, but using that very monster to do his dirty work. When a cult leader calls Jane out on all this, going so far as to tell him he's "coming along," Jane flatly denies it. And continues paddling along a river in Egypt.
In Life On Mars, 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, 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 Trailer Park Boys. 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.
Veronica Mars 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 Community, this happens to Abed when he is taking down a number of Alpha Bitches who humiliate other women. He ends up indiscriminately pointing out the flaws of everyone.
Antonia of True Blood 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.
King Uther in Merlin is a great example of this. 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.
Hmm—except he invented most of the 'monsters' in his grief, and his own Heel behavior isn't a mirroring of anything he was fighting. Some of his opponents might apply...it's not even clear how much of a bitch Nimue was before he went Rambo on her religion and all its adherents.
Granted, that still makes the show an example of the trope, since you can say that all of the total monster sorcerers Merlin goes up against are only that way because they're trying to get their revenge against Uther.
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.
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 Sympathetic Inspector Antagonist, 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. 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.
Robot Chicken does an extrapolation of Revenge of the Nerds 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.
Space: Above and Beyond gives us Col. Ray Butts, who apparently was born mean and became meaner from being a Marine lifer—he's racist against In Vitros, picks pointless fights with the Wildcards, antagonizes Mc Queen 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. 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 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.
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 Tracts 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?)
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.
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
In Japanese Mythology, 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 La Pucelle, this is the basis for a Non Standard Game Over, one that gets taken more or less as canon in the Disgaea series. And in 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 Saiyuki, where it's a part of Hakkai's backstory.
In Warhammer 40000, The Nighthaunter's backstory is made of this trope. Since he's the grimdark version of 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 theocraticdystopia that goes against the secular, rationalist utopia that their 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 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 Wide-Eyed Idealist (and that's saying something, given the setting and 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 Dungeons & Dragons, generally forming the backstory for antagonists, but sometimes providing players various forms of Anti-Hero to play.
Forgotten Realms has an elven god, Shevarash, dedicated to nothing more than hatred and vengeance to Lolth and drow (though he readily beats anything Always Chaotic Evil 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 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 Psychic Powers. 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 Complete Monster 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 taint further drives them to evil acts as they learn to fight their sworn foes.
Inverted with Dr. Rudolph van Richten of 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 sicced some zombies on a Vistani band and 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 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 Character Alignment.
In the Pathfinder Adventure Path Second Darkness, the Elven noble Allevrah Azrinae hunted the Drow with such passion and hatred that she turned into one (PathfinderElves do that).
Hunter The Vigil 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 Hunter The Reckoning, Hunters with very high Virtue ratings may become obsessed with that Virtue. Zealots (including Avengers, Defenders, and Judges) are particularly likely to become Ax Crazy and tolerant of collateral damage.
One of the subtler risks in Changeling The Lost, for many reasons considered the bleakest of the New World of Darkness games. As a Changeling's Wyrd (power meter, roughly akin to, say, 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 Clarity hits zero, though... they lose their grip on reality entirely and proceed to adopt the same Blue and Orange Morality as the True Fae, which is quickly followed by losing all traces of humanity and becoming a new member of The Fair Folk. 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 Legend Of The Five Rings, 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.
Seymour, the shmuck who feeds the man eating plant in Little Shop Of Horrors, 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.
Sweeney Todd is like the film example...only worse. 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...
King Kashue will quote this word for word to the Trestkon towards the end of the WorldCorp ending in The Nameless Mod.
2027: One of the possible outcomes in the epilogue is that the Judicians end up becoming those whom they have fought.
Ryu from Street Fighter. In the 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 Shadow Hearts face this trope. The downside to their Mega Manning 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 Warcraft III. This was apparently engineered (or at least taken advantage of) by the Big Bad, 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 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 Lovecraft-inspired Old Gods, the demons Sargeras corrupted and forced into his 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 'indeed nothing' after downing Illidan.
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 Warcraft III and World Of Warcraft.
After the death of the Lich King, 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 she is to becoming like the Lich King.
The Scarlet Crusade, the fanatical undead-hating organization, which has made the exact same journey as the Lich King/Arthas, have, as of Cataclysm, become undead themselves, because of Balnazzar's Villainous Breakdown
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. During the War of the Shifting Sands, his son, Valstann, was killed in front of him by the Old God-aligned Qiraji, which he 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.
And Too Human, which even gets its title from another Nietzsche work, Human, All Too Human.
And Legends of Valour.
Baldur's Gate 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 Puzzle Quest, the main character falls into this in the bad ending, effectively replacing the villain of the story.
The video game Phantom Brave features a character like this. His name is Sprout, and he knew that he was absorbing the Big Bad 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 Big Bad that were fusing with him. This Heroic Sacrifice weakened the Big Bad for the heroes to fight.
In Knights Of The Old Republic, 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 Knight Templar mode, talking about how a Jedi must "harden their hearts" and "do what is necessary" to fight the Order's enemies.
And 300 years later: The newly-freed and still-alive Revan (very long story) decides the best way to pay back the Sith Emperor for that And I Must Scream imprisonment is by coming up with an army of KillerRobots 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 Karma Meter on the first two games, and Nice Job Breaking It, Hero for the Republic players...
In Final Fantasy IV, the elder of Mysidia uses this phrase to warn Cecil that he could fall into this trap until he willingly sheds his darkness.
Nippon Ichi occasionally does this straight (particularly with 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 La Pucelle, because this is her CANONICAL ENDING.
Baby Bonnie Hood/Bulleta from Darkstalkers let her heart become so blackened in her quest to kill all the paranormal creatures that the Big Bad 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 officialsources say that she was actually born with a dark soul which makes her resemble Darkstalkers.
In Terminal Reality's awesome monster-hunting game 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 reanimated mobster Icepick or working side-by-side with the 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 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 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 Left 4 Dead: 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 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 Harmless Villain 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 Penumbra: Black Plague, after learning a big lesson about the Tuurngait being the original inhabitants of earth who liked mankind but hid because mankind was becoming violent, and completing some trials to show that you understand that the Tuurngait are not evil, you contact the outside world and reveal the location of the mine, and request that they Kill Them All. 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.
Blackwatch from 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.
Ace Attorney's Godot definitely qualifies. Starting as a 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 The Plan 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 Phoenix Wright himself becomes this in Apollo Justice. After having his attorney's badge taken away for presenting forged evidence, he loses respect for the whole judicial process, and manipulates crime scenes, forging evidence to get Kristoph Gavin convicted.
In Shadow of the Colossus, you end up absorbing the colossus essences, culminating in becoming a demon yourself. Granted, later on, you are reborn as a baby, but still...
Yuri himself skirts this trope and is well aware that he almost falls into this. Ironically the one who did fall into this is Sodia. Her own Vigilante Execution of Yuri basically made her into what she had always despised him for.
In 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. 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 Mega Man Zero, Mega Man X admitted to no longer caring about the enemies he fought. What's worse was that if it weren't for the Continuity Snarl between the two series, he would've become a full-out Maverick Knight Templar... X, however, was savvy enough to realize this and took himself out of the fight by acting as a living seal for the Dark Elf before he could go over the edge.
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 Mega Man ZX, the manner in which the Artifact of Doom, 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 The Virus. If fighting monsters (both literal and metaphorical) ultimately causes you to become a monster - with Model W 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...
Shadow Complex 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 Power Armor 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 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, 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.
Mass Effect has 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 and take him alive.
In the sequel, his loyalty mission has him tracking down the only member of his vigilante squad to live through an ambush (considering that he was The Mole), 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.
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 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.
Suikoden Tierkreis makes this recursive. You can take down the Big Bad the easy way by sacrificing the lives of all your allies, or look for another, harder way to beat him. 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 "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, you replace him.
In Dragon Age, the Grey Wardens are an order of warriors, rogues, and mages dedicated to fighting 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 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 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 Dragon Age II, 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 Well-Intentioned Extremist.
To be honest, the entire storyline is built on this. The leaders of both factions eventually give into their demons. Knight-Commander Meredith, who had fought blood mages all her life (okay, and an Artifact of Doom), 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 Blood Magic to put down Meredith and the templars who had oppressed them. They both jump off the slippery slope for what they see as the right reason. It really fuels the game's relentless Grey and Gray Morality.
Considering the back story of Killzone, the Helghast could be seen as these. Fleeing to a Death World after losing a war trying to resist the UCN's tyranny, they became as bad as the very people they fought.
In BlazBlue, Kokonoe has a seething hatred of Terumi Yuuki. In her attempts to bring him down, though, she has committed some acts that are earning her the ire of the fanbase, like treating 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.
The Patriots in the Metal Gear Solid 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, Paramedic, and Sigint ended up becoming corrupt, and eventually became the very thing that they attempted to remove.
The first preview of the Demon Hunter class from Diablo III seems to be this trope wearing a hood.
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.
City of Heroes 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 Heel Face Turn.
Subverted in 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 Marisa Kirisame possibly converting herself into a youkai in a bid for power, though it hasn't happened.
It's lightly hinted at in Kingdom Hearts II that 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 dozens of people inside your mind.
Word Of God says this outright, also stating that Anti-Form is a product of it.
DiZ/Ansem the Wise, who, in his efforts to make sure that neither The Heartless nor the Organization XIII takes over the universe, rely on kidnapping, psychological manipulation, and playing the You Have Outlived Your Usefulness card on his Nobody allies. At least he regrets it in the end before he got blown up and killed, for a while.
In 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 New Game Plus where 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.
Spec Ops The Line - 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. However, Walker does realize what he has done, through flashbacks and hallucinations
In 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.
Sins of a Solar Empire: 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 guns all over it.
In a very literal case, in Dark Souls, the Knight Artorias clashed with the dark monsters of the Abyss, earning the title of Knight Artorias the Abysswalker. 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 Bio Shock Infinite, the 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 in cold blood, before attempting to kill an innocent child on the grounds that they might grow up to be as bad as their parents. Even after 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 Castlevania Lords Of Shadow, 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, 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 kill Marie. The main game ends with Gabriel given a chance for redemption, but the Downloadable Content reveals that during his quest to contain the Forgotten One, 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 The Stinger rolls around, Gabriel has a That Man Is Dead attitude and proclaims, "Eu sunt Dracul!"
The Vatican and Aesir churches of Cry Havoc 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.
This is invoked in GastroPhobia: when Bambikles seeks to avenge his mother by killing the monster who took her life, he's told that "To kill a monster, you must become a monster." So he does.
Vaarsuvius became a very literal example of this trope but has since "recovered" and now regrets their actions.
Redcloak, depending on your interpretation. Although in his case, it's less a matter of fighting with (against) his enemies and more fighting with (alongside) Xykon.
Professor Broadshoulders from Zebra Girl 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 wanting to cure her condition to indulge her demonic hunger for pain and torment.
In The Gamers Alliance, 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 Global Guardians PBEM Universe, 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 Heroic BSOD 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 Survival of the Fittest 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 Anti-Hero 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 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.
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.
Movie Bob made a reference to this trope when he was describing the progression of the Shrek movies, pointing out that while the first movie was original and entertaining, the last two became that which they had been mocking in the first movie.
Discussed by Gordon Freeman in Freeman's Mind, 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.
Protectors of the Plot Continuum 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.
Depth Charge in Transformers: Beast Wars 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 Big Damn Heroes moments, but that doesn't excuse his not being there fighting in the first place. He also utters this trope's Stock Phrase word-for-word: "It's not revenge I'm looking for, it's justice."
In his second appearance, he was busy fighting with Rampage while the Maximals carried out a very important mission regarding the retrieval of the security system from their old base. After his fight with Rampage, he decided to check up on the Maximals and see if he could help, but ended up causing a blunder (thanks to the lack of communication) that lost the system to the Predacons.
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.
Megatron himself is this in several continuities, such as most of the comics series, Beast Wars, Transformers Animated and 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 anti-organic bias and somewhat psychopathic minions.
General Wade Eiling in Justice League Unlimited became so obsessed with taking down the show's titular League (and metahumans in general), that he injected himself with a Nazi-created Super Soldier 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 Lampshade Hanging on this trope at the end.
Ditto Jet, for that matter, although he calmed down a little eventually. And by "eventually", we mean "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 Sequel Series, The Legend Of Korra Amon 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.
On ReBoot, little Enzo Matrix never took the presence of Lawful Evil Megabyte too seriously, until Megabyte finally took over Mainframe, exiled The Hero, 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 gave Enzo a guitar for his birthday.
However, giving Enzo a guitar was more a 'Rule of Funny' 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 True Colors.
Agent Kent Mansley from The Iron Giant, 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 got a nuke launched at a small American coastal town. Where he happened to be.
In 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.
Aaahh!!! Real Monsters: Simon the Monster Hunter wanted to prove that monsters were real, and fancied himself the Only Sane Man, 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.
South Park loves this trope. In one particular episode, in order to keep 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).
It was implied that Cartman became such a Jerk Ass because he was constantly bullied in the 1st Season. Rewatching the earlier seasons and especially season 1 he was constantly being bullied.
The Simpsons 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. Lampshadesd 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 Bad Future, Gosalyn finds that her absence made Darkwing Duck turn into Darkwarrior Duck whose idea of justice is Disproportionate Retribution. He is, without a doubt, far worse than any of the villains who he has already disposed of permanently.
Sweetie Belle: Why does life have to be so ironic?!
The Powerpuff Girls 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 who finally realizes this, the girls becoming as bad as the villains they fight over a candy addiction.
Don't like it so much, now that the boot's on the other foot?