A Crown of Stars: In an earlier chapter Daniel explained to Asuka that he fell into this a very long while ago, and during that time he made a lot of things he was not proud of, and it was thanks to his family that he managed to stop acting like the monsters that he was fighting. He says it's why he now focuses Avalon's efforts on helping and healing, rather than bringing judgement on the wicked (they still do the latter to some extent, but that's not where their primary interest seems to lie).
A central theme in Avatar: The Last Airbender Revised is preventing oneself from succumbing to He Who Fights Monsters by only killing when absolute necessary. This is present primarily in Katara's character arc, although it fits into those of others as well. A paraphrase of Nietzsche's quote is used as the Arc Words for the first three books to reflect this.
Child of the Storm has averting this trope as an underlying theme, as Harry's becoming more open and better with his emotions means that his former Suppressed Rage is... not so suppressed. Plus, since he's the son of Thor, part of the House of Odin (which is known for the Warrior's Madness), it's potentially even more explosive. And to make matters worse there's repeated implications that his mother, a cousin of Jean Grey's, invoked the Phoenix as his protection. This, in concert with his powers developing faster than he can control and the sort of things he faces, from petty everyday cruelties like the bullying of Luna Lovegood and the Dursleys treatment of him to grander scale evil such as that perpetrated by HYDRA, mean that more than a few are genuinely frightened that he'll turn into the next Magneto if not watched carefully. They include Harry himself, who is downright terrified of this trope, to the point where it stunts the telepathy side of his Psychic Powers, since he's scared witless of what he might do with them.
Then he gets over it by the early parts of the sequel, Ghosts of the Past, becomes a bit more comfortable with it all, while strictly adhering to Mind over Manners... and then Sinister and the Red Room happen to him, torturing him and transforming his Blank Slate body into a killing machine akin to the Winter Soldier with superpowers. When he gets his body back, he unsurprisingly snaps, and it's far worse than anyone feared. A Roaring Rampage of Revenge takes him to the very edge of tipping over into the dark side of this trope. Specifically, by becoming TheDarkPhoenix. He is barely talked down in time, and entire pantheons live in fear of what'll happen if he snaps again.
Magneto, naturally, is referenced, with Wanda mentioning that she called him out for it and that he wasn't particularly pleased. However, it seems to have sunk in, since he's mellowed by the time of the story's writing, limiting his antics to occasional cases of Pay Evil unto Evil. Of course, the monsters are still terrified of him, and for damn good reason.
Dumbledore also goes out to avert this, avoiding seeking power on the grounds that he's perfectly aware that he could become a Dark Lord who would make Voldemort look like a fluffy bunny, in large part because of his deep frustration with the insularity of the British Wizarding World.
Doctor Strange regretfully remarks in Ghosts of the Past that in fighting monsters and manipulating history and people alike to craft what he deems to be the best timeline - specifically, the one most likely to survive the coming of Thanos - he has become something of a monster himself.
The Dee-Dee Morgendorffer (Daria's school nickname) of the long-runningDaria shared-world series The Hallowed Halls Of Fielding fits this trope like a glove. In a world of horrible students, an administration and parents that make looking in the other direction a honored tradition and corruption of everyone in the manner of a virus, Daria (a middle-class girl there because of her grandmother's footing the bill) decides that she's not happy with the way things are (even though the system's been working for a couple of hundred years). In the space of two years, she's caused a few disruptions so severe that they've literally changed the face of the school (and the society they service forever)...
Harry Potter and the Methods of Rationality plays with this trope. As early as the Sorting Hat, people have known that Harry could be a terrible force for evil if he turned. Dumbledore even considers an alternate interpretation of the prophecy, that the Dark Lord shall mark him as his equal, to be that Harry will become his successor. Meanwhile, Quirell himself seems to be actively attempting to use this trope on Harry. Even Harry is worried about it, given how far he's strayed to the Slytherin side of things since the beginning of the story, and how he refers to Hermione, Neville, and McGonagall as his consciences. However, while he fights with Slytherin tactics, and even declares early on his desire to become a god and rule the world, he and Voldemort have very deeply different goals, and Harry is disgusted by the level of cruelty displayed by Voldemort, and has never once displayed the same cruelty.
It's implied that this trope happened to Dumbledore and while he never became Voldemort, and still doesn't understand why Voldemort did the things he did, fighting Grindlewald brought him so close to the abyss that fighting Voldemort made him break several rules and do many things he regretted.
Not only the theme but the name of He Who Fights Monsters, a Rosario + Vampire story where Tsukune never meets Moka or the others. To keep anyone from finding out that he's human (and thus stay alive), he's eaten bugs, made several Improvised Weapons, and killed 5 students, including Inner Moka. "When your life's on the line, there's nothing that you won't do to survive."
This is also what caused Celestia to become Corona. She grew so obsessed with trying to protect her little ponies from all the dangers around them that she tried to control every aspect of their lives, becoming the Tyrant Sun. In the end, Luna had to use the Elements to banish her to the sun.
In The Hobbit fanfic The Monstrous Company of Thorin Oakenshield, Thorin set out to create a society better than the patriarchal one the Company, who are all female in this fanfic, have fled from. Some readers have commented that Thorin is rather close to becoming the monster, as males are only allowed into reconquered Erebor if they agree to be followed by armed (female) guards all the time. As of 2016 the fanfic is not yet completed, so it remains to be seen whether this will lead to becoming a monster, or stop at some point.
In Respect, this happens to YayoiKise. She makes a contract with Kyubey to stop bullies from tormenting her... and ends as a berserk supervillain, responsible for turning her home into a Ghost Town.
In Act I, Astreal dips into this territory when she decides to brainwash Rason and Dark to force them to fight Ghaldin for her and Apoch. When Apoch points out that they'd be stooping to Ghaldin's level by doing so, Astreal insists that it's not the same because Ghaldin used the two of them to kill whenever and whoever he wanted, while they won't use the boys for such actions.
In Act VI, Talon is dipping into this territory quite well. The members of Fairy Tale who were actually responsible for his transformation are already dead, but he doesn't care; as far as he's concerned, everyone who was ever a member of Fairy Tale, including Unwitting Pawns and those who left willingly, is equally guilty, even Dark, who was one of the heroes who took down Alucard. In Act VI chapter 35, Akua and Kahlua even flat-out tell him to his face that he's no better than those who made him what he is.
In a Digimon fan fiction Zero 2: Crisis on Infinite Digiworlds, one of the universe Davis travelled is a world where Kari is the only Digidestined left after taking down Davis's corrupt self Darkheart along with the absorbed Digidestined. Believing that darkness is evil and light is good, Kari began to systematically build hi-tokyo upon the old Tokyo, create a world where every single aspect of the people's life is controlled by the Queen of Light, unable to control any aspect of life, and completely seal any access to the Digital World in hopes to create a utopia where darkness and war will never happen. Unfortunately, this ended up completely stripping off humanity any of their free will, and judging from how her response when the old Tokyo is filled with Digimons hiding people that have not been purified be her light is to immediately purge them, this universe of Kari has become no better than the darkness she tries to extinguish and Azulongmon's claims that even light can corrupt is not exactly wrong.
In Kill Or Be Killed, the story begins with Leeteuk's death, with him serving as The Lost Lenore for Taeyeon. However, she becomes so hellbent on avenging her lover's death that she loses her sense of self to the point where she has to be killed to be stopped before she can become a big threat. Later in the story, Sunny's lover Sungmin is killed in front of her, and Sunny is so devastated that she almost commits suicide due to having nothing to live for. Her wants of avenging Sungmin's death almost turn her into a second Taeyeon to the point where, before dying, Yoona asks Yesung to tell her not to go down that road.
An aversion in Nosflutteratu's Garlic Flank Stake, who appears at first to be the typical "all vampires are evil and must die" vampire hunter but quickly changes her tune when she learns the vampire in question is Fluttershy.
The Open Door: To defeat the C'tan and Necrons, who plan to destroy all other life in the galaxy, NewChaos become The Unfettered and do some very questionable things.
Fujin (who became Earthrealm's protector god after Raiden went on his rampage) often worries if he'll end up just as monstrous as his fellow god.
Fade: With his own Death Note and (incomplete) knowledge of events, L seeks to stop Kira by any means necessary. By the story's midpoint, L has effectively become the Kira he sought to stop. Light, meanwhile, subverts this — while still a Serial Killer, he is nowhere near as bad as L.
A distinct worry for Suzume in Eroninja is that Naruto might not actually be a Corrupter and that she's simply jumping at shadows in her plan to expose and murder him. Her biggest moment of doubt comes when she strips naked and (not really) drukenly begs him to fuck her. Not only does Naruto turn Suzume down and cover her up, but the next day he seems even more mortified than she does.