He Who Hunts Monsters

"And I thought I was evil!"

- Plankton about one of Krab's act of crossing Moral Event Horizon that made him far despicable than himself.

"He Who Hunts Monsters", also known as "Falling Off Tte Slippery Slope," is a term used often to describe a phenomena in which a heroic or anti-heroic character becomes progressively worse as he or she fights percieved evils, to the point they become as bad (sometimes worse) than the evils they originally opposed.

The term has been populared by TV Tropes, but did not originate on that website, it has been around for a long time and is actually part of a famous quote by Friedrich Nietzsche that states: "he who hunts monsters must be mindful not to become one himself" - related to another warning of "stare too long into the abyss and the abyss stares back."

A great many heroes at some point undergo this transformation and it can be the key origin of many tragic villains, although rare due to it giving a character a Fruedian Excuse it can create Complete Monsters. In general however it is associated with Knight Templars and Vigilantes.

Examples

 * Punisher (his murder of criminals edges him ever closer to this. However, most depictions manage to avoid it by the fact he refuses to harm innocents - this can not be said for the versions found in more adult-themed works, where his sociopathy is even worse than in the "mainstream" comics)
 * Magneto (although a Holocaust survivor himself Magneto has often come close to parallelling some of the xenophobia and even genocidal tendencies found in the Third Reich - only instead of oppressing Jews he oppresses humanity (non-powered mutants) : most depictions of Magneto acknowledge this and fight against it, though some alternate versions of Magneto surrender to their darkness and become merciless oppressors of humanity)
 * Hulk (forever fighting with his own demons the Hulk also faces some brutal and sadistic enemies, as well as delusional adverseries who see him as a major threat even when he usually wishes only to be left in peace.. this conflict came to a horrifying conclusion with Maestro, which is basically what happens when Hulk decides to let out his aggression and hatred at the world)
 * Imperium of Man (although never truly "heroes" the Imperium of Man are examples of an entire society that have become progressively more brutal and oppressive due to internal strife, corruption and constant fighting against enemies far worse than themselves (Tyranids, Orks and Dark Eldar) )
 * Gordon Walker (He has viciously tortured and attempted to wipe out and murder Lenore and her nest despite being aware that they were only killing cattle and were genuinely trying to avoid harming humans, even goes so far to have conflict with Sam and Dean (in anime adaptation, he did it on a family of good vampires instead). This action ironically led to his transformation into the very monster he despised)
 * Aggie Prenderghast (In spite of her tragic past, she unknowingly became the very thing that worse that the Judge whom responsible for her death until Norman snapped her out of it)
 * Gamera: In Heisei era, though Gamera still fighting to protect the universe, Gamera's efforts in fighting the various threats includes Gyaos swarm sometimes edges him to this as he also indirectly caused destruction and deaths of innocent people and traumatized those whom survived his fight against evil kaijus. This was apparent in the events of Gamera 3: The Revenge of Irys where the destruction that left from his fight against two Gyaoses proved disastrous that not only causes Japanese Government to order Gamera's immediate destruction, but also causes Ayane, whom family accidently killed by Gamera himself in 1995, releases Irys, an abomination that proved to be worse than Gamera or his enemies.
 * Eugene H. Krabs: His rivalry with Plankton, and his worsened madness with money ultimately drove him insane as revealed in post-movie season (which set between events of Season 1-3 and the first film), where his actions in making money became as evil and extreme, to the point he become a murderer (though the only case where he performed act of murder was by slaughtering jellyfishes just to extract their jelly) and potentially destroyed his own daughter's innocence.