Usually, not quite a villain, but they act antagonistically enough that they're little better. Something has happened to our Fallen Hero: his village was destroyed, his friends killed, his puppy roasted on an open spit, his bike stolen, whatever. All that matters is that It's Personal, and he feels that the law just isn't suitable enough (or has become too corrupt and ignorant) to be of any use to him in settling the matter. He may justify his actions by claiming that it's Justice he's after, not vengeance, but anyone with half a brain can easily see that he's out for Revenge... unfortunately, we can also see that the more he hunts the cause of his woes, the more he takes on the villain's personality and mannerismssomething that our "hero" is too blinded by his single-minded goal to realize.
Our avenger may have good intentionsthe fiend may well be too dangerous to be kept alivebut ultimately, his obsession with dealing out due punishment (or worse) and his refusal to think about what he's doing twists him into a monster just as bad as, or even worse than, the one he's hunting. And even before he gets to that point, it's nigh-impossible to turn him away; calling him out on it will be ignored or retaliated against. The Power of Friendship and The Power of Love were lost to him the moment the atrocity that sent him on his wild goose chase happened; he feels that Team Spirit is just a hindrance, and that Love Is a Weakness that he can't afford to have. Heel Realizations will be ignored. Don't expect him to make a Heroic Sacrifice or HeelFace Turn anytime soon; if he dies in the process of bringing his nemesis down, it's usually with him crossing into Villainstown in his moment of glory. If he doesn't die...
The "fighting monsters" line represents what is a recognizable Moral Event Horizon for heroes, and both Anti-Heroes and Well-Intentioned Extremists live just near the boundary, especially the more pitiless Good Is Not Nice, Unfettered, Pragmatic Hero types. Engaging in Van Helsing Hate Crimes is a good indicator of having crossed the boundary and a Knight Templar is most certainly beyond it.
As expected, this twisted situation is very popular in the Revenge Tragedy genre, especially because of its inherent Dramatic Irony. note This trope can also be used to demonstrate how "eye-for-an-eye" justice, while sounding like sweet Karmic Equivalent Exchange Justice at first, can easily spiral out into utter chaos if the hero lets his passions, wrath, and Pride forgo rationality.
May be expressed through the page quote, or via the common misquote "When you become obsessed with the enemy, you become the enemy".
Not to be confused with Complete Monster, although somebody can well become one by fighting such monsters. They can range from Anti-Hero to anywhere above. See also Cycle of Revenge, The Dark Side Will Make You Forget, Protagonist Journey to Villain, You Are What You Hate, Then Let Me Be Evil, and Became Their Own Antithesis. If this trope happens to a child, it can be used as a Freudian Excuse of a character who Used to Be a Sweet Kid. Compare And Then John Was a Zombie, where the character becomes a literal monster. If You Kill Him, You Will Be Just Like Him is pretty much a sped-up version of this. Political equivalents are The Horseshoe Effect, Reign of Terror, Full-Circle Revolution, and Meet the New Boss. If the monster in question is an animal, that's Animal Nemesis. If the monster is supernatural, it's a decidedly unheroic instance of Hunter of Monsters. Subtrope of Slowly Slipping Into Evil.
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- Batman: Mask of the Phantasm: Alfred also refers to the Nietzschian quote, in regards to Andrea aka the Phantasm.
Alfred: Vengeance blackens the soul, Bruce. I've always feared that you would become that which you fought against. You walk the edge of that abyss every night, but you haven't fallen in and I thank heaven for that. But Andrea fell into that pit years ago, and no one, not even you, could have pulled her back.
- Batman: Under the Red Hood, like Phantasm, examines this Trope, only this time in regards to Jason Todd.
- Dreamworks Animation's Monsters vs. Aliens has an interesting inversion in General WR Monger, who spent 50 years as the warden for the titular Monsters, and ended up just as nice and eccentric as the monsters themselves.
- Agent Kent Mansley from 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.
- Justice League: Crisis on Two Earths, after Batman's Omnicidal Maniac Alternate Universe counterpart Owlman gives him a Not So Different speech, Batman's eventual rebuttal (slash Pre-Mortem One-Liner) references the Nietzsche quote at the top of the page.
Batman: There is a difference between you and me. We both looked into the abyss, but when it looked back at us... you blinked.
- Superman vs. the Elite: Superman shows how frightening he can be to people who wished he would cross this line.
- In Big Hero 6 Hiro attempts to use Baymax to kill Yokai/Robert Callaghan. However, both his team and Baymax help him defy this trope when they bring him back to his senses.
- This is what happened to Yokai/Robert Callaghan himself - his daughter's apparent death drove him to extremely desperate measures to take revenge on Alistair Krei.
- ParaNorman: The legendary witch was so intent on punishing those who wronged her that she became just as angry and spiteful as them. Norman calls her out for wanting to hurt people just because she's scared and angry, just like the townspeople did to her.
- Chirin of Ringing Bell is motivated by the senseless killing of his mother by the Wolf King. He goes on to become just as ruthless and bloodthirsty as the wolf.
- Bob Dylan's "My Back Pages":
In a soldier's stance, I aimed my hand
At the mongrel dogs who teach
Fearing not I'd become my enemy
In the instant that I preach
- While Dylan presumably was writing about the Author 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?)
- U2's "Peace on Earth":
They say that what you mock
Will surely overtake you
And you become a monster
So the monster will not break you
- Radiohead's "Bangers and Mash":
If you stare into the dark, the dark will stare back
back into your SOOOOOOOOOUUUUUUUUUUUUUL
- Atreyu's "Becoming The Bull"
Grab the bull by the horns the old adage goes
Nobody tells you where to go from there
Seems like fate's pulling you
Decisions have to be made
The best path is the hardest earned
Back and forth the struggle consumes us all
Trying to keep a level head
In the most unsettling of times
Today I'll become the bull.
- This happens at the end of "Strength of the World" by Avenged Sevenfold after the narrator went on a Roaring Rampage of Revenge on those who killed his family:
So far forever now alone, a greater punishment on me has been imposed
A killer falling from the light, I'll miss my family, I'll never be alright
- "Night of the Hunter" by 30 Seconds To Mars:
I was born of the womb of a poisonous spell
Beaten and broken and chased from the land
But I rise up above it, high up above it and see
I was hung from the tree made of tongues of the weak
The branches, the bones of the liars, the thieves
Rise up above it, high up above it and see
Pray to your God, open your heart
Whatever you do, don't be afraid of the dark
Cover your eyes, the devil's inside
One night of the hunter
One day I will get revenge
One night to remember
One day it'll all just end, oh
Blessed by a bitch from a bastard's seed
Pleasure to meet you, prepare to bleed
Rise, I'll rise, I'll rise
Skinned her alive, ripped her apart
Scattered her ashes, buried her heart
Rise up above it, high up above it and see
- "Murder" by Within Temptation and indeed most of the album The Unforgiving is about violently hunting down monsters:
I'm about to do it your way
I will make your world unsafe
I never thought you'd get this far
- Possibly implied in "Crawl" by Breaking Benjamin:
- 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 countryfor 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.
- Saiyuki's portrayal of this myth especially invokes this: while the various anime adaptations more or less consistently show that merely killing a thousand youkai turns one into a youkai, in the manga Hakkai's case is unique, and Word of God specifically states that it's not the act of killing or "being bathed in youkai blood" that triggers the transformation, but the killer's inner negative energy. In other words, this can only happen to those who already lost enough of their humanity and cultivated enough of their hatred that they might as well be compared to monsters themselves.
- "Stone Cold" Steve Austin's Heel Face turn during the WWE Attitude Era. After years of battling Vince McMahon and eventually Triple H who was with Vince, Austin out of desperation to become the WWE Champion again, does a We Can Rule Together and reveals he joined Vince at Wrestlmania XVII. Cemented during the following Monday Night Raw broadcast where Steve Austin publicly beats up his best friend Jim Ross who openly spoke against Austin's decision.
- Austin was on the other side of it a few years earlier during his feud with Bret Hart. Austin kept going after Bret, calling him out, and attacking him at every opportunity. Getting beaten by Bret in matches didn't discourage Austin, who kept hounding him. So it really shouldn't have been a surprise that Bret finally got angry enough to absolutely brutalize Austin in their famous submission match at WrestleMania, which was the beginning of his FaceHeel Turn.
- Dr Stevie became more and more violent in his efforts to deal with the violently insane. In some ways, he was worse than his patients, as they really couldn't help it but Stevie knew exactly what he was doing. Then again, since Stevie thought this approach would work, all evidence to the contrary, one could say he had become insane himself, albeit in a different way.
- Hinted at the end of Kevin Steen and El Generico's confrontation at Ring of Honor Final Battle 2010. Generico, after suffering a whole year of his former friend's abuse, has Steen on his knees and is holding the very chair that was used to start the feud a year earlier. As Generico swings it, Steen holds up Generico's former mask, as if to suggest actually hitting him would make him just as bad as he is. Generico holds the mask up to his face, contemplates it... And realizes that no, Kevin Steen is just a complete scumbag who deserves to get hit in the face. He drops the mask, smacks Steen with the chair, and pins him for the victory.
- Carly originally had no interest in becoming a pro wrestler and was happy to just help around the family business, working as a camera man in WWC. This changed when Ray González began targeting Carly because he was feuding with Carly's father at the time. 14 years later, Carlito Caribbean Cool is one of the top wrestlers in WWC and is targeting Ray González Jr, who till then was content to simply help with his father's mini market.
- In The Gamer's 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.
- Survival of the Fittest:
- Adam Dodd in 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.
- Older Than Steam: The Revenger's Tragedy starts when the Duke propositions Vindice's wife, and when she refuses, he kills her. And rapes her corpse. And what Vindice does, in the course of getting his revenge...is actually a hundred times worse.
- 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...
- Cyrano de Bergerac: when Cyrano gives us his Bully Hunter speech in Act I Scene IV, he has just bullied a poor Bore who only slightly bothered Cyrano...So Cyrano, according to the Law of Disproportionate Response, invokes the Berserk Button of his nose so he can dispense Disproportionate Retribution, kicking the Bores ass. Its obvious that Cyrano has been bullied before because of his enormous nose, but he is so badass compared to anyone else in the play that now he is the bully.
- In The Secret of Sherlock Holmes, the titular secret is that Sherlock Holmes is actually Professor Moriarty as well, an alter-ego to allow him a tight watch on the criminal world and to keep him from getting bored.
- Ace Attorney:
- 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.
- Phoenix Wright himself becomes this in Apollo Justice: Ace Attorney. After having his attorney's badge taken away for (accidentally) presenting forged evidence, he loses respect for the whole judicial process, and manipulates crime scenes, forging evidence to get Kristoph Gavin convicted. He gets better by Dual Destinies.
- Damon Gant used increasingly unethical methods (including murdering a subordinate and framing a 14-year-old girl for it for the sole purpose of blackmailing her sister into framing someone else) to increase his control over the investigative process and neutralize criminals who could not be convicted otherwise, such as Joe Darke.
- In the sequel to Ace Attorney Investigations, the final villain (Simon Keyes) was driven to commit a large array of crimes including kidnapping two minors and framing both of them for murder, killing a man with a falling hot-air balloon and arranging the murders of two other people, all so that the murderers would be caught and arrested because he had been a witness to a murder 12 years earlier but was silenced by the joint conspiracy of Blaise Debeste, Patricia Roland and the fake Di-Jung Huang and forced to go on the run in order to avoid being killed. Edgeworth even points out that his motive was a mix of justice and revenge but that he had become as bad as his enemies. At the end of the game, Edgeworth pledges to capture corrupt people in power before their victims become examples of this trope.
- Archer from Fate/stay night. The famous "Unlimited Blade Works" poem that he had supposedly written about himself even carries a message that is very similar in nature to the "He who fights monsters" quote itself (if you manage to decipher its incomprehensible language that is). It tells the story of a man who spent his whole life in an agonizing pursuit through self-sacrifice, to become the hero of his ideals, until he one day realized that it was an ideal that could never be met, that evil would always persist, that all his sacrifices had been in vain, and that he had ultimately only managed to turn himself into a killer without a cause. He was just another part of the evil in the world that he once had thought he was fighting. The man who wielded swords had thus lost his identity to the point where there no longer was a man, there were only his weapons. He was "a man made of swords".
- Kennyo from Ikemen Sengoku used to be a gentle, peaceful monk until the massacre of his monk brothers by Nobunaga Oda drove him to commit heinous deeds to avenge them. He's fully aware that he's become a monster himself in his Roaring Rampage of Revenge to take down the Devil King, but believes that dooming himself to Hell is worth it if he can drag down Nobunaga there with him.
- RWBY: The White Fang was once a peaceful protest group, trying to advocate peace between humans and Faunus, but five years prior to the start of the series, a faction within the group, feeling that said peaceful protests were getting them nowhere, staged a coup and took control of the entire organization, twisting it into a violent terrorist group who, by the time of the show's beginning, has become no better than the humans who persecuted and shunned them; even other Faunus find the White Fang's violent actions actions sickening.
- Salem also started out as a nice person until her husband succumbed to illness. Then she tried begging and lying to the gods to resurrect him, but they tortured her for her arrogance and made her immortal for her deceit. She committed suicide repeatedly for years, but nothing worked and she grew bitter at the gods. Then one day, she realized they could be fought, and ultimately beaten. Drunk on hatred and ambition, she rallied the nations to overthrow and replace the gods. After the gods laughed at their attempt, they showed their true colors by committing omnicide and justifying the murder of their creations as ending a mere experiment. Salem went insane from further attempts to end her life in a barren wasteland, culminating with the acquisition of divine power. Centuries later, she tried her hand at becoming a goddess herself, sadistic genocide included, only for her husband to divorce her and take the kids. She accidentally killed them all in a fit of rage, and she has been the sociopathic Grimm-ruling overlord of Remnant ever since, obsessed with summoning the gods so she can end them.
- 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 looking anything like the monsters he was fighting, especially not the fifty-foot tall Tentacle he was dealing with at the time.
- In The Adventures of Dr. McNinja, when the doc is subject to More Than Mind Control by an evil motorcycle helping him hunt down King Radical, this trope provides the "More Than" part.
Alt Text: To be fair, Doc always WANTED to blow up helicopters. The bike just gave him the means.
- Paranormal Mystery Squad: Stephanie is introduced as having a staunch zero-tolerance policy against cryptids, because of what happened to her parents. But by the time of the "Vampire Cheerleaders Must Die!" arc, she's come to accept that not all cryptids are evil as she first thought, as a result of her sister becoming a werewolf and after spending time with their friends, who're also cryptids. Stephanie later becomes a rare literal example and figurative subversion when she becomes a cryptid herself.
- Bittersweet Candy Bowl: After years of taking Tsundere-ish abuse from Lucy, Mike finally, shall we say, "gets her back" by not only turning her down, but eventually brutally and deliberately destroying any trace of their relationship, professional or otherwise. The problem is, Lucy has already gone through extreme Character Development by this point, so Mike's actions cause her to spiral into a deep, near suicidal depression, which makes him as bad, if not WORSE to her than she ever was to him.
- Oglaf plays this trope for laughs in that strip.
- When the second part of the phrase (the abyss part) was used in The B-Movie Comic, creator Roman Wunderlich declared in The Rant that he didn't understand what the big deal was:
"I never understood what's supposed to be so bad about that... I mean, I'd see that the abyss is deep, and the abyss'd see that I'm shallowbut it's not like I've been denying that, anyway..."
- Jenn of Casey and Andy just manages to catch herself doing this.
- 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 world's 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.
- The Order of the Stick: Vaarsuvius became a very literal example of this trope but has since "recovered" and now regrets their actions.
- 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 Pacificators, the reason regular people hates those with powers so much is because one of their best Pacificators ever, Castella Brandsford, went mad and murdered thousands of people before she was finally killed herself.
- In Girl Genius, the main reason that the Big Bad the Other, aka Lucrezia Mongfish, is so reviled is her use of mind-control via slaver waspif one flies into your mouth, you become susceptible to any order given by herone character is ordered to die, and does so immediately. Baron Wulfenbach is her main enemy, with the stated aim of destroying her and all her work, no matter what it takes. Which makes his personality overlay onto his son, Gil extremely hypocritical, especially since this is the exact same thing that Lucrezia did to her daughter Agatha. It's called out by the Big Bad herself in comic, that in trying to protect Gil from her influence he's ended up becoming just as bad as she is.
Lucrezia in Agatha: My goodness! How Mister High-and-Holy has fallen! Using your own son?
- TV Tropes: Some tropes come about as a deliberate attempt to defy a stereotype or cliché, only to become as over-exposed as the very thing they were defying. A good example is Real Women Don't Wear Dresses, which began life as an attempt to defy the stereotype of women as being weak and overly-sensitive, only to become a stereotype in and of itself that suggested that women who don't wear masculine clothing or act in a stereotypically masculine way are inferior to those that do.
- Fighting Monsters is named after this trope, and plays with it thoroughly, bringing up the original quote in the prologue, as well as throwing it at the main character in the form of a Wham Line.
- In Sailor Nothing, Himei worries that she is turning into this.
- Satirized by The Onion, "Little Boy Heroically Shoots, Mutilates Burglar".
- 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.
- 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.
- In the mission into Rainbow Factory, Caroline goes absolutely postal with the impostor Rainbow Dash, and later says she would probably have gone completely over the edge had Kilroy not hit upon the right word to bring her to her senses.
- Subverted by SF Debris, regarding Captain Janeway:
She has stared into the abyss as it has stared into her... and the abyss said "JESUS!"
- In Worm, Taylor begins to epitomize this trope as she gets deeper and deeper into the politics of being a cape, eventually becoming a Well-Intentioned Extremist who makes it clear that there are no lines that she refuses to cross if it accomplishes her goals.
- Cobra Kai gets hit hard with this trope, when Johnny sees the results of his dojo at the All Valley Tournament. Miguel, Aisha, and Hawk just wanted to be able to defend themselves against the students bullying them. While Johnny's training helped them achieve those results, it also led to them becoming the similarly ruthless, aggressive brutes that his own gang had once been. And he's like, Oh, Crap!
- Moviebob made a reference to this trope in his review of Shrek 4 while he was describing the progression of the Shrek movies, pointing out that while the first movie was an original and entertaining parody of corporate-driven animated movies, the series had slowly progressed into becoming the very same type of corporate-driven animated movie that the first film had made fun of.
- The Trope Namer is quoted in the teaser trailer for the Slender Man series Tribe Twelve. And, sure enough, the series shows signs of this. It's now inevitable (well, probably) as Firebrand, the Collective member that pulled a FaceHeel Turn (with the help of HABIT) has been revealed to be a future version of Noah, the protagonist.
- Starting to happen to Percy in Critical Role. He starts as the Only Sane Man in the party, but when he finally encounters the people who slaughtered his family, he snaps, and starts to torture prisoners amongst other things.
- This is apparently the reason for Shego from Kim Possible's FaceHeel Turn (at least going by her Lawful Stupid brother's version of events). More likely, she was fed up with their stupidity.
- 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." 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.
General Eiling: [about to throw a car onto Shining Knight] Superman and your Justice League are a threat to a safe and stable world.
Elderly Woman: [standing between Shining Knight and Eiling] Drop the act. You think killing Superman would make the world safe? Or killing this boy? Or us? Tell me, how many of us do you have to kill to keep us safe?
General Eiling: They're the ones I'm after, not you. I'm not the menace, metahumans are! Superpowered beings!
Stander-by: You're the only one around here with superpowers!
[Eiling stops, dropping the car]
General Eiling: ...All right, I've become what I hate, I'll give you that. But in the long run, you'll see I was right. You'll see you need the likes of me to protect you from them.
- And before that, the Justice Lords, the alternate reality counterparts to the league who fell to this after Lex Luthor became president and killed the Flash.
- Avatar: The Last Airbender features a lot of these.
- Jet was first established as a freedom fighter similar to Robin Hood, but was later shown to be completely willing to take some innocent lives in order to kill some Fire Nation soldiers. He calmed down a little eventually. And by "eventually", we mean "too late".
- Fong the Earth Kingdom general, was obsessed with triggering the highly destructive Avatar State in Aang by ordering his men to attack him. He even went as far as to endanger Katara's life, by burying her alive.
- Hama was an imprisoned waterbender who taught herself the dark art of bloodbending, allowing her to control the bodies of her victims in a rather painful way. She used this to escape the Fire Nation prison. In her later life she became at least as destructive and cruel as the fire benders she hated, as she used her bloodbending to abduct innocent Fire Nation citizens.
- Katara herself almost becomes this trope in "The Southern Raiders". In order to find closure to the death of her mother, she decides to hunt down the man responsible for it, going as far as to willingly use bloodbending, a technique she really did not want to learn in the first place. When she does find her mother's killer, she almost impales him with a barrage of icicles, but manages to restrain herself at the last minute.
- This is a recurring theme in the Sequel Series, The Legend of Korra, where each of the main antagonists stands for an ideal, but takes said ideal too far, resulting in them becoming the villain.
- Amon was the son of the mob boss Yakone and a young waterbender who was taught by his father to bloodbend. Eventually his father's abuse of both him and his brother Tarrlok, brought him the belief that bending was the root of all evil and needed to be ended. By the time the series comes around and by the end of it he has become just as cruel as any bender was.
- Hiroshi Sato also fits, wanting to eradicate bending to avenge the murder of his wife. However, his thirst for vengeance goes to the point where he loses all capacity for love and even attempts to murder his daughter who opposes him and his plans. She even tells him that his wife would hate what he's become.
- Korra herself came within spitting distance of this trope during season 2. After her father was arrested on charges of conspiring to kill Unalaq and sentenced to life in prison, Korra arbitrarily decided the judge was corrupt and chased him down after the trial, threatening to have Naga bite his head off if he didn't tell her what she wanted to know.
- Zaheer wanted freedom for everyone and saw all the government corruption, incompetence and tyranny as something that needed to be dismantled completely in order to provide that. While no one shed tears over his first victim, the tyrannical Earth Queen, he still began an uprising and had plans to assassinate every world leader to plunge the world into complete anarchy. While he disdained hurting innocents, he wouldn't hesitate to do it to further his goals.
- Kuvira was the result of Zaheer's uprising. Originally a brave and heroic woman, she desired the stabilization, safety and reuniting of the Earth Kingdom after Zaheer's uprising. The stabilization took three years to accomplish and she was determined to never let it happen again, nor let the circumstances that led to Zaheer to come back. So, she overthrew the outdated monarchy and established the Earth Empire, becoming an even worse tyrant than the previous Earth Queen (albeit with much more noble intentions). Korra revealing to her just how far she had fallen is what finally convinced her to surrender.
- 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.
- A major theme in the "City of Thieves" episode of Adventure Time, though it is used more for the Rule of Funny.
- Ben 10: Ultimate Alien: Charmcaster becomes this after her best efforts in resurrecting her father drove her to madness and insanity to the point where she became even worse than the previous tyrant who killed him in the first place. After her father calls her out on this, she gets better...mostly.
- 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). Episodes such as "Fatbeard" display his borderline murderous vendetta against him.
- In earlier season it was implied that Cartman became such a Jerkass because he was constantly bullied for his weight. "Cartman's Silly Hate Crime 2000" goes so far to imply that Clyde almost underwent the same evolution after the boys turned their attention onto him instead, even taking on some of Cartman's obnoxious attributes.
- The episode "Butterballs" depicts anti-bullying advocates as being no better that those they are fighting against. Or the people that eventually advocate against them...and those that advocate against them...
- Kyle gets this most, considering his personal hatred for Cartman (not that Cartman's actions make it any less justifiable). Episodes such as "Fatbeard" display his borderline murderous vendetta against him.
- 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. (Homer, however, interpreted it as "Never put down your weapon.")
- Lampshaded in "Sex, Pies and Idiot Scrapes," where Homer and Ned become bounty hunters and Homer scolds Ned for "not becoming as bad as the people they were hunting."
- After time travelling to a 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.
- The seeming moral of the animated short "Who's Afraid of Mr. Greedy"
- This whole trope was the moral of the My Little Pony: Friendship Is Magic episode "One Bad Apple." Whether or not it was done so successfully is a discussion for elsewhere.
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.
- Any rare Tom and Jerry short that Jerry lost to Tom, this trope was often his undoing, being so set in his vendetta against Tom he'd attack him beyond provocation or even draw the first blow.
- 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.
- SpongeBob SquarePants:
Plankton: And I thought I was evil!
- In the pre-movie seasons, Mr. Krabs was originally the Big Good to Plankton's Big Bad, simply defending his business against Plankton's Evil Plans. Post-movie, due to Flanderization, he's often shown to be just as evil and cruel as Plankton, if not worse, and often makes it his personal duty to sabotage Plankton when he's making legitimate profits or simply isn't miserable. In "The Krabby Kronicle," Plankton himself is actually disgusted at how low Krabs would stoop to make money:
- Compared to Squidward, who openly hates SpongeBob and doesn't bother hiding it, Mrs. Puff was a docile, well meaning character whose nerves have slowly been worn at by Spongebob's antics. Several episodes have shown her take increasingly desperate and sometimes unscrupulous measures just to finally get away from him and in one episode, she outright tried to murder him.
- Bismuth from Steven Universe is this due to her making a weapon designed to kill her enemies (Homeworld Gems) with has plans to commit genocide against them and will destroy anyone who stands in her way (even allies), thus becoming as bad as the Homeworld Gems she fights against who have done the same against their enemies and allies.
- Frequently lampshaded by Kion in The Lion Guard. He was bestowed with the Roar of the Elders, which is given to the second-born cub, as the first is destined to be the king, while the 2nd is tasked with leading the Lion Guard, a group dedicated to preserving the kingdom and defending the Circle of Life. As its leader, the second-born cub is expected to be the fiercest of the group and is thus gifted with the Roar of the Elders by the Great Kings of the Past. Kion knows that Scar, Simba's grand-uncle, was bestowed with the Roar when Mufasa was heir, but used it for evil purposes, causing the Great Kings to strip Scar of the roar. Therefore, Kion's greatest fear is to become Not So Different from Scar, and it is this reason that forces him to regulate his use of the Roar.