Discussed extensively within Attack on Titan, both by the narrative itself and the characters. Leaders such as Erwin Smith and Dot Pixis embody the belief that throwing away human compassion is necessary to win against the Titans, ruthlessly sacrificing the lives of their soldiers for even the smallest chance of victory. Armin comes to believe that throwing away one's humanity is the only way to win against monsters, while Jean expresses to him doubt over whether such methods are truly worth it. Mikasa has honed herself into a ruthless fighter for the sake of Eren, even going so far as to threaten the lives of her own comrades. On the other hand, Eren is a far more literal example: becoming a Titan in order to fight against the Titans, but also struggling to embrace the ruthless actions others ask of him. Finally, there are the other Titan Shifters, who play with this trope in an interesting fashion: their own humanity comes back to haunt them, causing them to make critical mistakes. Annie spares Armin, and finds her mercy repaid with being exposed and trapped by the military. Reiner is unable to cope with the guilt of his actions, suppressing his memories and creating a false persona in order to function. Bertolt internalizes considerable remorse and self-loathing, unable to act on his own even to prevent Reiner's Sanity Slippage and when his own emotions boil over, he makes reckless mistakes that allow Eren to be rescued. Finally, Ymir fell In Love with the Mark, and is successfully blackmailed using her feelings for Krista.
Later in the story, Eren realises he's not so different from Reiner, Bertolt and Annie, going so far to telling Reiner that they/their ideals are the same, and they're carrying out their plans/attacks for the same reasons. He doesn't entirely seem to mind, however, so long as he achieves his goal of wiping out the enemy (and no, it's not titans anymore, as such), and he's accepted that that's his role in life.
Seems to also be the case with both Eren Kruger and Zeke Yeager, who each claim they're trying to save the Eldian people, but have only ever been shown attacking and killing Eldians (and turning them into titans, the very thing they're trying to stop) — with no apparent guilt. Granted, it's hard to know if they're telling the truth (especially in the case of Zeke), but the idea seems to be that to protect the ones you love you have to become a monster and throw away your humanity in order to succeed, perhaps so that you don't feel any amount of guilt to hold you back. Kruger brought up that love is important in fighting your enemy, and that you mustn't forget it, which could be referencing this trope, but it's left somewhat vague.
Eren then becomes the harshest and possibly most painful example possible. Eren began the series as a ruthless, but well meaning soldier dedicated to avenging humanity by getting rid of all the Titans. After a massive battle against his enemies, as Eren finds out in the worst way possible they aren't truly alone in the world, and the rest of the world utterly hates them for the atrocities the Eldians committed for centuries, even when Eldians were ultimately defeated and subjugated, Eren decides to fix this problem by commanding the remaining Pure Titans aka, the Wall Titans to commit mass genocide on the rest of the world.
In Berserk, Guts is every bit as cruel and sadistic as the Apostles he hunts (towards them at least) at the beginning of the series. He deliberately tortures the Baron and Count in the beginning chapters, and, after fighting with Rosine, doesn't stop trying to kill her even after an innocent girl who was tagging along threw herself in front of him. Currently, the only thing preventing him from completely going off the deep end is 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, thus able to harm things such as ghosts that are normally incorporeal and unable to take damage from normal weapons.
And he still comes across as much better than his adversaries, which showcases how bad they are. He is slightly more fettered than them and reluctant at hurting others first.
The PV for the anime beginning in July 2016 makes the theme explicit by actually quoting Nietzsche's "He who fights monsters" passage from Beyond Good and Evil, in the original German no less!
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.
Rock himself, throughout the series, but particularly during Roberta's Blood Trail where he develops a sadistic side due to Revy and Roanapur's environment, manipulating major players in the city and gambling with peoples lives, including Garcia, a mere child. Fabiola even states that Rock is now considered by her to be "the biggest number 1 bastard in Roanapur".
Bleach gives us Kaname Tousen, who became obsessed with justice when a friend of his was murdered by her Shinigami husband who managed to get away with the crime, even if said obsession entailed helping the Big Bad become a god and not really worrying about the countless innocent lives he was killing or planning to kill along the way. It eventually transpires that the reason he turned against Soul Society was for revenge (he has previously implied it was because Utopia Justifies the Means, though this was still somewhat true), believing that forgiveness of any sort was a mockery of justice and feeling he was completely entitled to take revenge on Soul Society and all of its inhabitants.
In one of the early cases of Case Closed, "The Art Museum Owner Murder Case", the main characters visit a the Beika Art Museum, where (of course) a murder happened to have taken place shortly before. Before the corpse is found, the characters discuss a certain painting showing a knight covered in blood from a demon he had just slain. After the case is solved, the curator gives another interpretation of the painting: "The Knight of Justice did indeed slay the demon, but was splattered with the evil blood and eventually turned evil himself." This is also used as a parallel to the case, as the murderer, who considered the victim a sort of "monster", ended up becoming a monster himself.
Even after his semi-Heel–Face Turn, Accelerator in A Certain Magical 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 considers fits "the hero" type.
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.
Suzaku Kururugi at the beginning appears to be a subversion. For most of the first season he condemns Zero and the Black Knights as nothing but terrorists that are causing unnecessary death and violence, and proceeds to battle them and climb the military ranks while avoiding killing anyone (usually because when faced with the dilemma of his superiors ordering him to kill, circumstances rescue him from going through with it). But come the ending of season one, after Euphemia is killed by Lelouch, Suzaku is not only willing to kill to achieve his goals but season two has him involved with Britannia’s expansionist war that sees him conquering whole nations.
Suzaku also qualifies in his quest of changing Britannia’s system, as in the beginning he attempted to be an Internal Reformist against the racism and corruption within the government, but as things progress due to the above incident, he simply does whatever it takes, whether it’s aiding in Britannia’s conquests or lying to and betraying his friends, in order to climb the ranks to simply have the power to help his home country.
In Cross Ange, Jill, leader of the Norma of Arzenal and Libertus, once cared deeply about her subordinates, but since the failure of the first Libertus, she had become cold, ruthless and manipulative, and only cares about killing Embryo, and will use anyone as pawns, and keep secrets and trust nobody else. Ange immediately grows to distrust her, and when Jill tries likening herself to Ange (both disinherited princesses) after attempting to use her maid and closest friend as collateral to follow her plan to use everyone else as a distraction, Ange angrily rebuts her by calling her as bad as everyone they're fighting. Turns out the first Libertus failed because Embryo raped Jill in both mind and body. Jill comes out of this after being forced into fessing up and having her leadership relinquished.
Ian deals with this in A Cruel God Reigns. He struggles with being angered at Jeremy's failed attempts to recover from being a Hooker with a Heart of Gold and Functional Addict and falling in love with him, but he worries that if he acts upon either of these, he will become like his Archnemesis Dad who caused all of Jeremy's problems in the first place.
Kyosuke Munakata in Danganronpa 3: The End of Hope's Peak High School is obsessed with winning the war against Ultimate Despair to the point where he declares Makoto a traitor and tries to have him executed for wanting to rehabilitate them instead of killing them outright. After Makoto explains his actions in Danganronpa 2: Goodbye Despair, Munakata declares him unworthy of the title of Ultimate Hope and again tries to kill him on the grounds that his naivete would lead himself and others to death and despair while ranting that he'd be willing to kill thousands for the sake of ending Despair.
Death Note: 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 intends to eliminate other "useless" people who aren't outright heinous criminals through accidents and disease. 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. Ryuk even lampshades this, wondering aloud if Light will ultimately be the only bad person left, but he's just in for the spectacle.
Dragon Ball GT has the Tuffles. They were a peaceful people (according to Baby and King Kai), who were almost completely wiped out by the Saiyans (who may or may not have been previously enslaved). So they created Baby, who plans on turning the entire galaxy into Tuffles. Nice, you went from genocidees to genociders.
Dragon Ball Super: Zamasu of the Future Trunks Saga starts out a Well-Intentioned Extremist who wants to protect the universe from the constant cycle of mortals fighting wars and destroying each other, but ends up becoming just as bad, if not even worse than the mortals he sought to protect the universe from. Present Zamasu tries to murder his own master so he can begin his quest to kill all mortals and Future Zamasu is helping Goku Black wreck Future Trunks' world. Goku Black (an alternative version of Present Zamasu) goes as far as to murder Goku's entire family after taking his body and then goes on to kill all the gods in the 12 universes.
It's been stated that those who use too much Dragon Slayer Magic take the risk of becoming a dragon themselves. Acnologia was once a Slayer who wanted revenge on the dragons, but after transforming into one himself his list expanded to the point he wouldn't mind if he just wiped out the rest of the world too in the process. Irene Belserion was a proponent of human/dragon relationships and made Dragon Slayer magic to fight the human-eating ones, but her own transformation and the fear and hatred her people treated her with (coupled with the trauma of realizing she could never again be human) turned her into a sociopath.
Devil Slayers like Gray aren't exempt from this either, as Invel said that Devil Slayers will eventually become the demons they fight due to The Corruption. Gray himself almost becomes this when he tries to kill Natsu/E.N.D., and when he snaps out of it he's so horrified he tried to perform a Heroic Sacrifice to take down Zeref.
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 noted to 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.
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.
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 prevent her from going down his brutal way.
Ray Lundgren from GUN×SWORD. At least the protagonist Van, while skirting this line, still had standards about not ruining property, taking innocent lives, etc. Ray, on the other hand, will gladly kill anyone, innocents included, if they stand between him and The Claw. He even claims that, as long as he can kill The Claw, he doesn't mind becoming a murderer himself.
Subverted when he wastes his only chance of killing his nemesis by purposefully missing his last shot in order to foil The Claw's plan once and for all.
In Mobile Suit 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 Awesome Music 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 Mobile Suit 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.
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. Spoiler aside, Anderson fully embraces the trope because to him he is merely answering God's calling.
Anderson: I am but a blade, wielded from on high: a guillotine blade in the service of God.
In the Meakashi chapter and by extension the Watanagashi chapter of Higurashi: When They Cry, 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 because she realizes that she's gone too far to deserve redemption, hence her tearful shock when Keiichi forgives her anyway and shows more interest in saving others than his own life.
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.
Howl, from Howl's 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.
Kurapika from Hunter × Hunter seems to be turning into this - his clan, the Kurta clan, was wiped out by the Phantom Troupe, and as such, he has dedicated himself to hunting them down (the sole exception being Hisoka, who wasn't a member at the time), but his methods increasingly feel like they wouldn't be out of character if they were performed by one of the Troupe.
In Kaleido Star, Yuri Killian is convinced that his boss Kalos killed his father indirectly in order to achieve fame. During his long revenge scheme, he needs to win a competition in order to stay at Kaleido Stage and prove his stardom to Kalos. In doing so, he is indirectly responsible for the death of another performer, Sophie Oswald.
Akoya Seishu of Kengan Ashura is a police officer and a martial arts master who dedicated his life to "justice" and became a vigilante who slaughtered criminals with his bare hands. Over time he slowly became a psychotic Knight Templar and extended his brand of "justice" to his victims' families and any rival fighters unlucky enough to get in his way. He beats his Morality Chain Hiyama for not giving him the results he wants as his Mission Control, and Adam Dudley accuses him of having "Dirty Harry Syndrome".
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 Kirby. Meta Knight reveals that he was the one who killed his father and calls him a monster for the awful acts he committed.
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 against 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'.
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.
Moriarty the Patriot has William, who was abused as a street orphan as a child, which definitely informed his lack of faith in anything resembling a justice system. So he's decided to kill abusers instead of hoping the cops will fix it.
Naruto is centered a lot around revenge, mainly on Sasuke Uchiha'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 eventually realizes that he was manipulated by Obito after he listens to the First Hokage's story about Madara and his dreams for the village. To make matters worse, this is complete with Karin being Ok with the fact he tried to kill her immediately afterwards.
Not that it really made a difference. He instead turns into a psychotic revolutionary who wants to kill the Kages, Naruto and the Tailed Beasts and create a new world order. So Naruto has to fight him one last time. But when Sasuke finally hears from Naruto's own mouth that he still believes Sasuke is worth saving, he steps away from the brink long enough to dare Naruto: if you want to save me, Bring It!
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 Kurama's attack 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. Though this could be seen as a Sadistic Choice given to him by Danzo.
Hanzo of the Rain village is also implied to have fallen to this trope; in his youth, he was a 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 Pain. Nagato's vengeance was horrifying, and when he was done, not only was Hanzo dead, but his entire family and any person that had any kind of relationship with him, no matter how tangential.
Naruto Uzumaki 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. Until Pain seemingly kills Hinata in front of Naruto. In his rage Naruto takes his Biju-mode even further than ever before, transforming first into 6-tails and then 8-tails mode, and is on the verge of willingly freeing Kurama entirely just so he'll kill Pain for him when his father Minato's spirit intervenes to stop him.
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.
Now and Then, Here and There: Poor Elamba. He was so devoted to killing King Hamdo he became as demented and twisted as he was.
Rob Lucci's belief in "Dark Justice" makes him just as brutal and violent as the criminals he hunts; Sanji and Franky even lampshade it after seeing Lucci brutally execute Nero for failure, outright declaring the so-called "agent of justice" to be more evil than they are.
Admiral now Fleet Admiral Akainu in particular needs to be mentioned. His very first appearance is in a 20-year flashback where he, then a Vice-Admiral, orders the destruction of a massive ship of innocent refugees because there might, just might have been one of his targets on board. For God's sake, even Blackbeard has more empathy than this man.
Not just him, but the World Government as a whole. Shortly after Gold Roger was executed, they found out that he had a child with an unknown young woman. Their solution? hunt every young mother or soon-to-be-mother they believed to be suspect, and kill them and their children. Pretty damn cruel. Not to mention it didn't even work anyway: the young woman who actually was pregnant with Roger's child held off on giving birth for 20 months, until the hunt had stopped, leading to her Death by Childbirth.
Shogun Kurozumi Orochi's entire character is built around this trope. His whole family was persecuted by some citizens of Wano for the crimes of a few members. Even their innocents. So, he wanted revenge, and when he came to power, he persecuted the entire country of Wano, even the innocents.
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 Gem 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.
In the Rebellion StoryHomura, who had spent many timelines fighting Kyubey to prevent Madoka's contract, ends up hijacking his plan (and literally becoming a Luciferlike figure) to circumvent the Law of Cycles Madoka created.
Ramen Fighter Miki: 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.
Ryu's Path: Both Condor and Medornote the talking dog that Ryu encounters in front of the crystal note that in trying to defend his friends, Ryu has become just as violent as those who he claims to fight against. Even Ryu agrees to this.
Ryu:"Monster"....Condor said the same thing. You're right, Medor, it's true. Without realizing it, I've turned into a monster. I didn't want this...I just wanted to help my people.
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.
In Tokyo Ghoul, several Ghoul Investigators are shown to have become just as sadistic and cruel as the worst Ghouls. In particular, the revenge-driven Kureo Mado takes pleasure in the suffering he inflicts on Ghouls and is responsible for some of the most vile actions within the series. When he corners the gentle Ryoko Fueguchi, he delights in showing her the weapon he has made from her murdered husband and taunts her before using it to kill her. Later on, he uses one of her arms as bait to lure in her 14-year old daughter — this time he has weapons made from both parents, and taunts the little girl with them extensively.
After deciding that hurting others is the only way to protect the people he loves, Ken Kaneki slowly becomes just as brutal as the Ghouls he considers to be monsters. He engages in Monstrous Cannibalism to increase his strength as quickly as possible, and even threatens to torture people for information on multiple occasions. It takes him nearly killing his friends to make him realize how far gone he is.
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.
Vinland Saga: Askeladd is half-Welsh, half-Norse and hates the destruction and loss of life the Viking Age has brought to the British Isles. In attempting to find a way to end the Viking Age, he became a viking mercenary commander and eventually took part in the Danish-Saxon War, committing atrocity upon atrocity. Unlike most examples he is fully aware that he's long since been tainted by his own deeds: when someone who can actually build a better world asks why he didn't try himself, Askeladd essentially responds that he was too far gone long before he had the power to change anything, and any world he'd build would be tainted by his own evil. Askeladd eventually dies this way, assassinating Sweyn Forkbeard before dying at Canute's hand to make sure Canute will ascend to the Danish crown and stop the bloodshed.
In the original Yu-Gi-Oh!, this was the reason for Kaiba's ruthlessness: he and his little brother Mokuba had been abandoned at an orphanage, and subsequently adopted by the very ruthless Gozaburo. In order to survive and take over his business, Seto adopted his adopted father's cutthroat tactics. After he loses the Battle City tournament and orders Mokuba to prematurely begin the detonation of the island, Mokuba gives him a much needed What the Hell, Hero? over how he changed for the worse.
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.
Aporia is like this in Yu-Gi-Oh! 5Ds. After surviving the Bad Future where humanity was wiped out by the Meklord Emperors, he tried to Set Right What Once Went Wrong, but barely cared about how many innocents had to die in the process, becoming as much a machine as the things that had caused him so much grief and using the Meklords himself in his goal.
Yu-Gi-Oh! ZEXAL: While not obsessed with revenge, Shark was unaware that the Barians he were fighting were his own people and that he is in fact their leader. In the end, he joins them.
In YuYu 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.
