In George Orwell's Animal Farm the animals initially intend to make an equal society but the pigs make small changes to the laws until the pigs are indistinguishable from the humans they once hated.
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 against mere suspects. Trials presided by him tended to be Kangaroo Court and some accused Death Eaters (Sirius Black, for example) weren't even granted any, or the benefit of doubt, before being left to rot in Azkaban.
Nightfall Series: Myra fears she can’t defeat the vampires without becoming as evil as them.
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 defense, he didn't have a way or an incentive to tell the difference. 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.
They 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. 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 of The Wheel of Time 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, as well as anyone who disagrees with them.
And Rand himself almost invokes this trope, until he subverts it gloriously at the end of The Gathering Storm.
In the backstory, roughly two thousand years before the main plot happens, a man named Mordeth decided that he would take the fight to the Shadow. Feeling that he needed great power to do so, he went looking for supernatural forces not related to the Dark One and found... something that gave him potent non-channeling based magic. He used his knowledge and power to gain great influence and did indeed oppose the Shadow, but unfortunately the... something turned out to run on The Power of Hate. Long story short, Mordeth's city-state of Aridhol ended up destroying itself through sheer hate which manifested as a killing fog called Mashadar, Mordeth himself became some sort of undead spirit doomed to linger in the ruins, and when he finally escaped by merging with a new body, he continued to oppose the Dark One - but in the process had become a supernatural evil in his own right, one most people consider just as bad.
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 Legends works—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 in Shatterpoint... A quote attributed to Yoda is a paraphrase of the passage from Nietzsche: "When you look at the dark side, careful you must be… For the dark side looks back."
This is also what leads Anakin Skywalker to become Darth Vader. In the novelization of Revenge of the Sith, Anakin and Obi-Wan fear this of Mace Windu, due to his heavy distrust of Palpatine and advocating for more direct action against him. Towards the end, once the Council starts planning for a possible Sith attack, Mace himself realizes how far he's gone.
At the end of the Warhammer 40,000Grey 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.
Alan Blunt, head of MI-6 in the Alex Rider series, is willing to engage in a number of questionable practises in a bid to get reluctant teen spy Alex Rider to go on the missions Blunt wants him to undertake. He finally crosses the Moral Event Horizon in Scorpia Rising when he stages a school shooting (injuring one of Alex's friends) in a bid to convince him to do one last mission. At the end of the novel, when he is confronted by his second-in-command Mrs Jones over his actions, he recites the Nietzsche quote.
The Kingpriest from Dragonlance started out as a Messianic Archetype, 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 StateSec 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 over the course of the trilogy. Asriel moreso since Mrs. Coulter was a power-hungry monster from the start. 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.
The Scoia'tael, brigades of nonhumans (elves and dwarves) that resist what they see as man's occupation of traditionally Elder Race lands can also count as this. Though they portray themselves as freedom fighters, they are extremely brutal, constantly attack civilians, and get non-Scoia'tael nonhumans in trouble due to suspicion that any of them could be a sympathizer. Some of them even attack non-Scoia'tael elves and dwarves.
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 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.
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.
Katniss and some of the other Hunger Games victors appear to be this when they decide to hold another Hunger Games with Capitol children in order to get revenge for the districts' suffering. It is actually part of a batman gambit to fool President Coin into thinking she's on her side.
The protagonists often worry about how to defeat their enemies without becoming just like them.
Alloran, who committed genocide to prevent the Hork-Bajir from falling into Yeerk hands, is said to have been an idealist before the Yeerks's betrayal and slaughter of his comrades drove him over the Despair Event Horizon.
Rachel became this. Before the final mission, she accepts the fact that she's become something of a nutcase and thinks that Jake's made the right decision in using her appropriately. To illustrate: she's been harboring worries about whether she could even function without the war anymore, and it's all too clear she is not exactly a paragon of mental health, even among the Animorphs, who have all spent the last three yearsholding the line, fighting a guerrilla war against a powerful alien empire. It's even possible that she's been insane at least since the events of the last book she narrates, The Return, after being horrendouslyMind Raped by David, who was working as a pawn for Crayak, to get revenge.
Depending on your interpretation, Jake's actions in the final arc, particularly the penultimate book of the series, may place him here as well.
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 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.
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 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 this trap herself, but manages to retain her humanity due to outside help.
The Egyptian freedom fighters in Oblivion, to Richard's disgust.
Lucian Clemant, Gerry Habbentz, Colonel Zak Larraine, and Prebendant Balthazar Delastro in The Lamb Among Stars. The interstellar Assembly is the Millennial Kingdom prophesied in Revelation, but when evil re-entered (also as prophesied) those four, among others, quickly sank to the point where they would do anything to defeat Lord-Emperor Nezhuala and his Dominion, even to the point where they meant to slaughter everyone in the Sarata system, the seat of the Dominion. Zak has a Heel Realization (though his final fate is unknown), Gerry is shot attacking Merral after they are exposed, Clemant commits suicide while being taken to the Moon, and Delsastro turns traitor and is Dragged Off to Hell even as good triumphs.
In the Monstrumologist series by Rick Yancey, the quote is recited (at least once in the original German, no less) by multiple characters. It's a constant worry and very visible struggle throughout the series, appropriately enough considering that the main characters' profession is quite literally fighting (and studying) monsters, and at least one character has indeed become this: Dr. Warthrop's "friend", Dr. John Kearns, who freely admits that's not his real name and is insinuated to be Jack the Ripper. Most of the other characters are still in the struggling/debatable zone, but everyone who's gone up against one of the monsters has shown some behavior reminiscent of this trope to some extent or another.
Fëanor and his sons from The Silmarillion. They are fighting Morgoth but in the process engage in kinslayings of other Elvish groups due to the Oath of Fëanor, for which they are cursed. By the end Fëanor and six of his sons are dead and the only survivor is unable to return to Valinor due to the Oath.
Saruman from The Lord of the Rings. He begins as leader of the Wizards who were sent to fight Sauron. Eventually, from studying the ring and power of Sauron he decides to join them, intending to get the ring himself to rule Middle-Earth. In this case it is more pride and intolerance than any revenge or anger at the monsters.
It is claimed using the ring against Sauron would cause this to happen due to the ring's corrupting influence. It leads to Boromir attacking Frodo to try to get the ring, before a What Have I Done reaction.
A major theme in the Doctor Who novel "Engines of War". The Time Lords are shown becoming no different than their enemies the Daleks during the Time-War, as they decide to wipe all the Daleks in the Moldox system, and arrogantly place themselves above billions of people, who also will die from the drop of the bomb.
A Symphony of Eternity has Metternich a generaly nice guy, who's a civilian drafter into the fleet.He broke down and cried during the first battle depicted in the series, after another he had almost a completer mental breakdown and after one hellish battle in which he was informed that his surprising tactical victory actually resulted in a strategic disadvantage for his side, he actually halucinates from the shock of it all and imagines himself bashing his commander's head into the ground. A far cry from the character who is his backstory he actually tried to save the lives of dying pirates that robbed him and beat him up for fun. War Is Hell indeed!
When Terahnee fell in the Myst spinoff novel Myst: The Book of D'ni, many of the (abominably treated) slaves followed the slave Ymur, whose primary goal was the total extermination of the Terahnee and their P'aarli stewards. In the end. he is starting to set himself up as a new Master, and the slave child Uta kills him.
Averted in Brightly Burning. After Lan accidentally kills a gang of older bullies who had tied him up and whipped him, he begins to fear that this makes him a monster. A friend who is studying law explains that extenuating circumstances exist for a reason and self-defense does not make Lan a murderer. Indeed, Lan grows up to become one of Valdemar's greatest heroes. They both still "feel bad" that the situation happened at all, but guilt no longer cripples them.
In Victoria, ex-military protagonist John Rumford and his allies are constantly teetering on the brink of this in their struggle for survival in a post-apocalyptic America, if not indeed overstepping the line at times. Their enemies range from despicable to nightmarish, but their own methods of defeating them are also extremely brutal, up to and including indiscriminate use of weapons of mass destruction.
Discussed, to great length for a childrens book, in The Brothers Lionheart. Orvar, one of the two leaders of the resistance against Tengil, is a good man, but fanatical in his hatred of everything Tengil stands for. In the end, Jonathan (the elder of the two titular brothers) chooses to go up against Katla alone, just as much to end Katla's rampage as to ensure that Orvar can't get his hands on the magical artefact Tengil uses to control Katla, since that would inevitably turn him into Tengil 2.0.
There is also the fact that Tengil is constantly referred to by his underlings as "our liberator", which makes one wonder if Tengil was one of these himself at one point. The book is silent on the matter.