History HeWhoFightsMonsters / LiveActionTV

** In ''KamenRiderZeroOne, Aruto Hiden witnesses [[spoiler: the death of Izu at the hands of Horobi]], which sends him into a HeroicBSOD and SanitySlippage so bad that he ends up [[spoiler: becoming Kamen Rider Ark-One, making him become the successor to the BigBad that's been responible for [[MonsterOfTheWeek Monsters of the Week]] that's been plaguing this series, with Aruto going on to ignite a CycleOfRevenge and putting Horobi through what he's been through.]] Notably, this is something ''Aruto himself'' acknowledges after his first transformation.

* One of the core themes of the ''Franchise/KamenRider'' franchise is the concept of "does becoming a monster means losing your humanity?" Almost every series has its Kamen Riders' powers come from the same source as the villains in their show [[spoiler: with some even crossing over the line and becoming the very MonsterOfTheWeek they've been fighting all along.]]

* One of the core themes of the ''Franchise/KamenRider'' franchise asks if becoming a monster means losing your humanity. Also every show and series has its Kamen Riders' powers come from the same source as the villains they fight [[spoiler: with some even crossing over the line and becoming the MonsterOfTheWeek they've been fighting all along.]]

** [[{{Irony}} Ironically]], the Centauri themselves (whose RPG rulebook even starts with Nietzsche's trope-naming quote). Before first contact with the Xon, the other sentient race of their own homeworld, they were peaceful artists who had even rejected the very concept of war. Then a naval expedition reached the Xon lands, causing the Xon to find out about them and attack the Centauri, killing and enslaving many of them. By the end of the war, that also included a brief alien invasion from the Shroggen, no Xon was alive, and the Centauri were a fledgling empire ruled by a DeadlyDecadentCourt and bent on expansion to get even with the Shroggen and protect other races. With time, they forgot their motivation.

** The two-part episode *Homefront/Paradise Lost* features [[spoiler: Admiral Layton, whose concern that the Federation wasn't taking the Dominion seriously enough leads him to fake a Dominion attack, attempt a coup, and order one Starfleet ship to fire on another.]] Luckily, Sisko manages to get in his way.
** *In the Pale Moonlight* has Sisko resort to several underhand tactics [[spoiler:and eventually condone a murder]] in order to coerce the Romulans to ally with the Federation against the Dominion. The final scene of the episode has Sisko chillingly coming to the realization that he can, in fact, live with what he's done.
** *Extreme Measures* has Bashir commit a form of Mind Rape against Section 31 operative Luther Sloan in order to save Odo's life.
*** Sloan himself (and his organization, Section 31) also fall under this trope, doing pretty much whatever they want so long as it's under the pretext of protecting the Federation.

** Antonia was a witch who was raped and murdered by vampires in the middle ages. When she comes back as a spirit, the next logical step is to attempt genocide against the entire vampire race, attacking and imprisoning everyone that stands in her way.

** It featured [[TheProfiler an FBI agent who became so obsessed with "getting into the head" of a serial killer that he was chasing]] that he soon turned evil and began murdering people in a manner similar to that used by the serial killer. Well either that, [[MaybeMagicMaybeMundane or he was possessed by the same demon that made the other man a serial killer]]. This was Mulder's former boss, and Mulder wound up having to stop him. He ended up being a BrokenPedestal for Mulder and other agents.

*** The Season 3 episode "Gingerbread" begins with Buffy's mom finding two young children after what looks like a magical rite. She responds by organizing the other parents in Sunnydale into an organization to go after witches (and Slayers.) The episode ends with them all trying to burn ''their own children'' at the stake.

** [[spoiler: Jack Bauer]] takes this trope UpToEleven in the second half of the eighth season, when he gets his hands on a murderer. [[spoiler: Jack eventually backs down when he realizes what the consequences (to innocent people) will be if carries out his revenge, stopping short of falling as far as Tony did]].

** The Minbari in general and their Warrior Caste in particular. After the last war against [[AbusivePrecursors the Shadows]] they spent a thousand years to prepare for the next, and just as it was coming [[PoorCommunicationKills a screwed-up first contact with Earth caused the death of their political and religious leader]], prompting them to start a genocidal war in spite of the humans trying to surrender multiple times. They stopped and surrendered right after destroying the last of Earth's military and a few minutes before actually enacting the genocide, thanks to [[spoiler: finding out evidence that Minbari souls are reincarnating in humans]], but, partly because the motivation was kept from the public, it takes a while for the Warrior Caste to stop behaving like everyone is beneath them (the first time we see a Minbari warship in the series, it repeats the same mistake that caused the war. Thankfully Delenn was there to explain that custom).

** Sheridan himself falls victim to this, when he suddenly encounters a survivor of the accident that he'd previously believed his wife'd died in. In a mad hope that if this man, Morden, is alive, then she might be as well, Sheridan unlawfully locks him up and interrogates at length, dismissing his own crew's objections. Furious by Morden's refusal to cooperate and suspecting him of lying [[spoiler:(he ''is'' lying)]], Sheridan threatens to keep him locked up indefenitely and even ''torture'' him.

*** Faith's character arc embodies this, presenting her as a [[EvilCounterpart dark mirror]] to Buffy. Faith is shown to not only slay demons, but to enjoy it 'a little too much' and she is very brutal about it. This was partly because her Watcher was murdered by a demon, but also because she resented anyone having power over her.

*** Holtz is so obsessed with obtaining "justice" against Angelus that he followed him into the future, disregarded all the myriad evidence of Angel's reformation, and did all he can to make Angel suffer psychologically. Although, at the end, he seems to make a comeback when he mentions that love has overcome hate. This turns out to be a ruse; he even uses his own death as further fuel to get Connor to take his revenge for him.

* In ''Series/{{Community}}'', this happens to [[SoapboxSadie Britta]], [[BewareTheNiceOnes Shirley]], and [[TheCutie Annie]] when they use [[AmbiguousDisorder Abed's]] BrutalHonesty to take down a number of [[AlphaBitch Alpha Bitches]] who humiliate other women. They end up indiscriminately pointing out the flaws of everyone, until it reaches the point [[MetaGuy Abed]] invokes HoistByHisOwnPetard by giving the former {{Alpha Bitch}}es the perfect insults to use on them.

** Between "[[Recap/DoctorWhoS1E2TheDaleks The Daleks]]" and "[[Recap/DoctorWhoS12E4GenesisOfTheDaleks Genesis of the Daleks]]", it is all but stated that the Thals were the initial aggressors in the long war, and that over the generations the Dals[=/=]Kaleds went from being the good guys to being [[ANaziByAnyOtherName the greater of two evils]]. And that was even before Davros even went as horribly wrong as he did.
* A mayor plot point in both seasons of Argentinian HBO crime series ''Epitafios'', appearing in season 1 [[spoiler: with Renzo, who murders Costas in cold-blood after his murder spree (including Laura)]] and in season 2 it comes back with a vengeance with [[spoiler: both Marina and Renzo, the former shooting her brother's murderer and the latter burying the main villain of the season... ''[[MoralEventHorizon alive]]'']].

** In his youth, Robert Baratheon led a rebellion to depose the cruel and paranoid Mad King Aerys II, but as king he resorts to increasingly unsettling means to keep his own dynasty on the throne and to keep said king's family from reclaiming the throne, though, unlike Aerys he does realize that he's going too far, and tries to call off his hit on Daenerys on his deathbed.

** Peter is also headed down this path in Season 3 of ''Heroes'', when [[spoiler: taking Sylar's power in order to save the world caused him to also gain Sylar's hunger]].

* One season 9 episode of ''Series/LawAndOrderSpecialVictimsUnit'' had a serial killer found dead in the same manner as his victims. [[spoiler:Turned out, it was the lead investigator who killed him, because her mentor committed suicide from the stress of trying to catch him. However, it meant that she inadvertently killed his last victim, who had been abducted but not killed yet. Though she initially plays it cool, it eventually becomes clear that, even though the man she killed was a monster, she knows she crossed the MoralEventHorizon in doing what she did and she can't live with herself over it. When Olivia and Lake come to her apartment to arrest her, she tearfully quotes the page title word for word before blowing her brains out.]]

* ''Series/{{Lost}}'': considering who the series's ultimate BigBad is, this can be inferred as the reason for much of the Others's villainous behavior.

* In ''Series/TheManInTheHighCastle'' this is a general theme for the resistance. While it doesn't apply to ''all'' of them all the time, in order to keep their fight up they have had to make moral sacrifices. The most prominent example is [[spoiler:George Dixon. Dixon]] wants nothing more than to see America free from Nazi rule. [[spoiler:to achieve that end he spies on people using the Nazi's own surveillance lines, even other people he is suppose to be on the same side with. He is willing to kill any Nazi for his cause, even the civilians. He plans to have the senior most Nazi officer killed by his own people by revealing that his son has an incurable genetic illness, even though that will kill an innocent child in the process. Juliana says he is just as evil as the Nazis and his response is that they must be eviler to win and then justifies his actions by arguing that the boy he would kill is sick anyway. He dies in a Nazi uniform, a disguise he was wearing at the time, when Juliana refuses to let him do this.]]

** Merlin himself. During the course of the series, he has constantly lied to hide his magic, committed countless murders and on one occasion betrayal, and can be just as ruthless as his arch-nemesis Morgana. If not for his loyalty to Arthur, he could go to the very deep end.

** FBI Special Agent Donnelly is an interesting example. Being a SympatheticInspectorAntagonist, he seems to be a generally capable, upstanding lawman who is chasing what he is sure to be a well-funded terrorist group (In reality, Team Machine). However, over the course of the series his methods become more and more extreme, to the point of labeling four men who, as far as he knows, could just be bankers as terrorists and holding them against their rights, and even going so far as to almost allow a possibly innocent man to be beaten to death by prison inmates (hoping he would reveal his combat training). Eventually, he stops trusting anyone and loses the respect of Carter. [[spoiler:Donnelly finally gets his man due to his Paranoia, but because he assumed the situation was less complicated than it was, he ended up dead for his trouble.]]

* In ''[[Series/SpartacusBloodAndSand Spartacus: War of the Damned]]'': A number of the rebels are showing signs of this, as seen when they slaughter innocent civillians including children. Spartacus wants them to be better than the Romans, but is unable to keep them in line. Gannicus is aware of what they are becoming, but seems to have resigned himself to the inevitability of it.

** Gordon Walker is the purest example, becoming worse than the monsters he hunts taking them out. For a series that can succumb to the temptation of explicitly spelling out character psychology as frequently as ''Series/{{Supernatural}}'' (how many times has someone told Dean that he lacks self-esteem, is afraid of being alone, is dead inside, yadda yadda yadda), Gordon was thankfully handled with restraint. In his three individual episodes, he comes off as just a sadistic bastard, but put them together and the story is all there: his family blamed him for letting his sister disappear (they wouldn't believe that she had been vamped), and he [[StakingTheLovedOne hunted her down and killed her]], refusing to admit that it was out of anger instead of necessity. But inside, he is so guilt-ridden that he is desperate for everyone to see the world in terms of black-and-white (which would justify his actions), with Gordon on the side of the good guys (thus his creepy obsession with getting Dean's approval).

** Dean was like this this after John died and he had that big-secret-that-totally-wasn't weighing on his shoulders, and has had such moments of ruthlessness every time his family leaves him or lets him down or he's really freaking out about his brother. Such as when he encounters Gordon in season two after his father dies; when he so loses faith in his brother that he agrees to the angels' plan in season five even though it will destroy most of the world; and in season seven when he [[spoiler:kills Amy Pond ([[Series/DoctorWho not that one]]) because he can't trust a monster not to kill again, complete with a [[Film/KillBill Beatrix Kiddo]] moment with the woman's son afterward.]]

* Most of the Argent family from ''Series/TeenWolf''. Their role seem to be keeping supernatural creatures in line, but can be just as cruel as the werewolves. Chris Argent is more of a KnightTemplar, but has no qualms about threatening sixteen-year-olds. Victoria is fine with torturing ordinary humans that do not even know werewolves exist just to create job vacancies for Hunters. Kate and Gerald were each an outright psychopathic ManipulativeBastard.

* ''Series/VeronicaMars'' implies that Keith and Veronica's career choices are starting to take their toll on the characters's well being and sense of morality.

* ''Series/{{Highlander}}'': The Dark Quickening is this. It's what happens when a good immortal takes in too much evil from the others they defeat and corrupts them.

*** Dark Willow started out wanting revenge against Warren for what her did to her girlfriend, Tara, then moved onto his friends because of guilt by association, and soon escalated into CardCarryingVillain territory, until she eventually [[OmnicidalManiac tried to destroy the world]]. Willow was brought back from the abyss by [[ThePowerOfLove Xander]].

* ''Series/{{Arrow}}'' gives us [[ComicBook/{{Huntress}} Helena Bertinelli]], who is initially shown as a WoobieDestroyerOfWorlds type because her father killed her fiancee. Naturally, she wants revenge on her father. However, her plot involves [[MoralEventHorizon killing and maiming multiple innocent people.]] LikeFatherLikeSon at its worst.

* In the ''Series/StarTrek'' episode "The Savage Curtain", the Excalbians notice that both the "good" team and the "evil" team used the same tactics. Kirk explained it was ''the reason'' that they fought, the Enterprise crew was threatened if the good team didn't fight while the bad team was offered "power" if they won.

* ''Series/{{Merlin}}'':

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** This theme is frequently explored in the revival series, especially in the Twelfth Doctor's Series 8 character arc: He's unsure whether he's "a good man" because of all he does for the sake of saving the day. In [[Recap/DoctorWhoS34E2IntoTheDalek "Into the Dalek"]] a malfunctioning, dying Dalek has switched sides, but when it's repaired, it returns to its old ways. The Doctor [[spoiler:melds his mind with "Rusty" to show it the beauty of life... but it sees how much the Doctor hates the Daleks. That hatred is so strong and, to a Dalek, beautiful that it once more is willing to destroy its own kind. Rusty even claims that the Doctor is the truly "good Dalek" in this situation, calling back to the Ninth Doctor's [[Recap/DoctorWhoS27E6Dalek similar encounter]].]]

** Here's a capper: In the ''ComicBook/DoctorWhoTitan'' comic "The Swords of Kali", the Twelfth Doctor paraphrases the trope-naming quote to warn Rani Jhulka about the dangers of seeking revenge... and then adds "Should '''never''' have given that quote away [to Nietzsche]. Could've dined out on it all across the universe."
** The Doctor has spent a ''lot'' of time fighting monsters and staring into the abyss, to the point that it's been stated (and demonstrated) several times that part of what keeps the Doctor being his best self is his companions. We've seen him be pulled back from the brink of going too far by his friends, and when he doubted he was a good man, ''Clara'' never did. "Don't be a warrior — be a Doctor."

*** Sloan himself (and his organization, Section 31) also fall under this trope, doing pretty much whatever they want in their role.

* One season 9 episode of ''Series/LawAndOrderSpecialVictimsUnit'' had a serial killer found dead in the same manner as his victims. [[spoiler:Turned out, it was the lead investigator who killed him, because her mentor committed suicide from the stress of trying to catch him. However, it meant that she inadvertently killed his last victim, who had been abducted but not killed yet. When Olivia takes her back to her apartment to get the gun, she tearfully quotes the page title word for word before blowing her brains out. However, she herself was actually a subversion -- her only victim was an unrepentant psychopath; it was heavily implied that, had he been caught, he would've misled the cops and caused them be too late to save his last victim anyway just ForTheEvulz; and, given the time frame, her torture-caused gangrene was most likely too severe to be survivable anyway. Even the guy's own mother felt that the investigator did more good than harm.]]

* In ''Series/CriminalMinds'', the Nietzsche quote is used three times, once in the first episode, once in the hundredth episode,and once in the two hundredth episode. It's referenced in the season four finale during the finale voiceover ("How many more times will [my team] be able to look into the abyss"). However, the BAU doesn't really fit this trope, and, in the hundredth episode, [[spoiler: it's pretty clear that Hotch did the right thing]]. However, Gideon's departure from the team is due to his fear and realization that he's been staring into the abyss for too long and can no longer see humanity past it. He leaves to wander the world for a while and restore his faith in humanity.

* ''Series/GameOfThrones'':
** In his youth, Robert Baratheon led a rebellion to depose the cruel and paranoid Mad King Aerys II, but as king he resorts to increasingly unsettling means to keep his own dynasty on the throne and to keep said king's family from reclaiming the throne, though, unlike Aerys he does realize that he's going too far, and tries to call off his hit on Daenerys on his deathbed.
** Daenerys believes so firmly in SlaveryIsASpecialKindOfEvil that she views any punishment she inflicts on the slave masters as justice.
* Most of the Argent family from ''Series/TeenWolf''. Their role seem to be keeping supernatural creatures in line, but can be just as cruel as the werewolves. Chris Argent is more of a KnightTemplar, but has no qualms about threatening sixteen-year-olds. Victoria is fine with torturing ordinary humans that do not even know werewolves exist just to create job vacancies for Hunters. Kate and Gerald were each an outright psychopathic ManipulativeBastard.
* Eben from ''Series/TheSecretCircle'' seems to have become this from fighting John Blackwell.
* ''Series/TheXFiles'', episode "Grotesque":
** It featured [[TheProfiler an FBI agent who became so obsessed with "getting into the head" of a serial killer that he was chasing]] that he soon turned evil and began murdering people in a manner similar to that used by the serial killer. Well either that, [[MaybeMagicMaybeMundane or he was possessed by the same demon that made the other man a serial killer]]. This was Mulder's former boss, and Mulder wound up having to stop him. He ended up being a BrokenPedestal for Mulder and other agents.
** When Mulder was working on this case, he was sinking into the darkness as well, because he also needed to get into the killer's head. Both Scully (his partner) and Skinner (his current boss) were extremely worried. Ultimately, Mulder subverts the trope - he was messed up, but he got better once the case was resolved.
* ''Series/BuffyTheVampireSlayer'':
** Dark Willow started out wanting revenge against Warren for what her did to her girlfriend, Tara, then moved onto his friends because of guilt by association, and soon escalated into CardCarryingVillain territory, until she eventually [[OmnicidalManiac tried to destroy the world]]. Willow was brought back from the abyss by [[ThePowerOfLove Xander]].
** The whole concept of a "Slayer" is based on this. A Slayer is supposed to fight demons, but her powers are demonic in nature. She is not expected to be nice to those whom she runs into in life and her life is short, nasty, and brutish. Several episodes (the one involving the First Slayer and an alternate universe version of Buffy, among others) deal with this.
** Faith's character arc embodies this, presenting her as a [[EvilCounterpart dark mirror]] to Buffy. Faith is shown to not only slay demons, but to enjoy it 'a little too much' and she is very brutal about it. This was partly because her Watcher was murdered by a demon, but also because she resented anyone having power over her.
** The Season 3 episode "Gingerbread" begins with Buffy's mom finding two young children after what looks like a magical rite. She responds by organizing the other parents in Sunnydale into an organization to go after witches (and Slayers.) The episode ends with them all trying to burn ''their own children'' at the stake.
** This literally happened to Forrest.
* ''Series/{{Angel}}'':
** Holtz is so obsessed with obtaining "justice" against Angelus that he followed him into the future, disregarded all the myriad evidence of Angel's reformation, and did all he can to make Angel suffer psychologically. Although, at the end, he seems to make a comeback when he mentions that love has overcome hate. This turns out to be a ruse; he even uses his own death as further fuel to get Connor to take his revenge for him.
** Angel himself goes pretty far into this territory in season 2, and he seems to do it deliberately, re-shaping himself into someone willing to use evil methods to wipe out evil.
* ''Series/DoctorWho'': The TechnicalPacifist Doctor has killed ''very many'' Cybermen and Daleks. He has annihilated various monsters of the week and entire fleets of enemy spacecraft, as well as, presumably, his own people. The Doctor seems to swing back and forth on the SlidingScaleOfIdealismVersusCynicism quite frequently. In one case, he was attacked by creatures who wanted to steal his immortality. [[AndIMustScream They got their immortality all right]]. Getting the Doctor personally angry is, in his own words, "not a good place to stand." This theme is frequently explored in the revival series, especially in Twelfth Doctor's Series 8 character arc: He's unsure whether he's "a good man" because of all he does for the sake of saving the day. In "Into the Dalek" a malfunctioning, dying Dalek has switched sides, but when it's repaired, it returns to its old ways. The Doctor [[spoiler: melds his mind with "Rusty" to show it the beauty of life... but it sees how much the Doctor hates the Daleks. That hatred is so strong and, to a Dalek, beautiful that it once more is willing to destroy its own kind. Rusty even claims that the Doctor is the truly "good Dalek" in this situation, calling back to the Ninth Doctor's similar encounter]].
** For a non-Doctor example, the Rutans from [[Recap/DoctorWhoS15E1HorrorOfFangRock "Horror of Fang Rock"]] may be this in their long war with the Sontarans.
** Here's a capper: In the ComicBook/DoctorWhoTitan comic "The Swords of Kali", the Twelfth Doctor paraphrases the trope-naming quote to warn Rani Jhulka about the dangers of seeking revenge... and then adds "Should '''never''' have given that quote away [to Nietzsche]. Could've dined out on it all across the universe."
** The Doctor has spent a ''lot'' of time fighting monsters and staring into the abyss, to the point that it's been stated (and demonstrated) several times that part of what keeps the Doctor being his best self is his companions. We've seen him be pulled back from the brink of going too far by his friends, and when he doubted he was a good man, ''Clara'' never did. "Don't be a warrior - be a Doctor."
* Happens in the episode of the ''Series/TwilightZone'' "The Mirror". In this episode, a rebel overthrows a dictator in a banana republic. However, the dethroned dictator says the rebel will learn the consequences of ruling by force (i.e. killing people to maintain power). The new ruler becomes more and more paranoid, using more and more vicious measures to maintain his rule, proving he indeed became just like the dictator he deposed.
* ''Everyone'' in ''Series/{{Supernatural}}'' has this problem ''all'' the time. It's not just the contact-with-evil, that is, the 'Monsters' part; it's also the 'Hunts' part, the ''violence'' inherent in the lifestyle. Most (if not all) hunters are this, being pushed into hunting after having a loved one murdered by one of the monsters, which leads many to be obsessed with revenge.
** Gordon Walker is the purest example, becoming worse than the monsters he hunts taking them out. For a series that can succumb to the temptation of explicitly spelling out character psychology as frequently as ''Series/{{Supernatural}}'' (how many times has someone told Dean that he lacks self-esteem, is afraid of being alone, is dead inside, yadda yadda yadda), Gordon was thankfully handled with restraint. In his three individual episodes, he comes off as just a sadistic bastard, but put them together and the story is all there: his family blamed him for letting his sister disappear (they wouldn't believe that she had been vamped), and he [[StakingTheLovedOne hunted her down and killed her]], refusing to admit that it was out of anger instead of necessity. But inside, he is so guilt-ridden that he is desperate for everyone to see the world in terms of black-and-white (which would justify his actions), with Gordon on the side of the good guys (thus his creepy obsession with getting Dean's approval).
** All the Winchesters have been like this (mixed in with that good old DeathSeeker attitude) at some point. John was this way about everything related to Mary's death.
** Dean was like this this after John died and he had that big-secret-that-totally-wasn't weighing on his shoulders, and has had such moments of ruthlessness every time his family leaves him or lets him down or he's really freaking out about his brother. Such as when he encounters Gordon in season two after his father dies; when he so loses faith in his brother that he agrees to the angels' plan in season five even though it will destroy most of the world; and in season seven when he [[spoiler:kills Amy Pond ([[Series/DoctorWho not that one]]) because he can't trust a monster not to kill again, complete with a [[Film/KillBill Beatrix Kiddo]] moment with the woman's son afterward.]]
** Sam was this after Dean died in ''Mystery Spot'' and the season three finale. While he thinks killing Lilith is the only way to prevent the Apocalypse and feeding [[PsychoSerum demon-blood]]-fueled powers also lets him save the hosts when exorcising demons, his obsession with gaining the power to kill Lilith [[spoiler:leads him to break the final seal, releasing Lucifer from Hell]].
** Future Dean in "The End" (5x04). After losing his brother and failing to stop the apocalypse, he becomes heartless and unsympathetic, willing to sacrifice all of his loyal friends for a chance to kill Lucifer.
** In seasons 6 and 7, [[spoiler:re-angelified Castiel]] has taken a particularly nasty route to this, starting with a DealWithTheDevil, moving on to murder and betrayal, and then JumpingOffTheSlipperySlope with murder and MindRape of friends even ''before'' diving into WithGreatPowerComesGreatInsanity.
** Not even taking into account [[spoiler:Dean]] over his season 9-10 story arc, with the season 9 finale taking a rather literal interpretation of this trope.
** [[spoiler: Both Sam and Dean]] hit the peak of this in the final episodes of Season 10, with [[spoiler: Sam]] going to every length [[spoiler: he]] can to save [[spoiler: Dean]], regardless of the collateral damage involved or how badly [[spoiler: he]] manipulates everyone else, while [[spoiler: Dean]] slowly succumbs to the influence of [[TheCorruption the Mark of Cain]], eventually resulting in him massacring the people who killed [[spoiler: Charlie]] and anyone associated with them. The opening episodes of Season 11 have them having a HeelRealization after they released [[spoiler: [[PrimordialChaos the Darkness]].]]
* Eric van Helsing from ''Series/YoungDracula'' is a comedic version of this.
* ''Series/{{Dexter}}'':
** The title character, a VillainProtagonist, inverts this, as his fight makes him ''less'' of a monster than he would be otherwise. He's more a combination of EvilVersusEvil and EvenEvilHasStandards.
** [[spoiler: ADA Miguel Prado]], however, falls into this after his brother is killed, leading him to find out Dexter's secret and learn his methods, leading him to [[spoiler: [[MoralEventHorizon kill defense attorney Ellen Wolf]],]] and [[spoiler: attempting to murder [=LaGuerta=], before Dexter kills him off.]]
* A mayor plot point in both seasons of Argentinian HBO crime series ''Epitafios'', appearing in season 1 [[spoiler: with Renzo, who murders Costas in cold-blood after his murder spree (including Laura)]] and in season 2 it comes back with a vengeance with [[spoiler: both Marina and Renzo, the former shooting her brother's murderer and the latter burying the main villain of the season... ''[[MoralEventHorizon alive]]'']].
* ''Series/{{Heroes}}'':
** HRG. While much of what he does is for Claire, working to capture the monsters in Level 5 shaped him into the unscrupulous operative he is today
** Peter is also headed down this path in Season 3 of ''Heroes'', when [[spoiler: taking Sylar's power in order to save the world caused him to also gain Sylar's hunger]].
* One season 9 episode of ''Series/LawAndOrderSpecialVictimsUnit'' had a serial killer found dead in the same manner as his victims. [[spoiler:Turned out, it was the lead investigator who killed him, because her mentor committed suicide from the stress of trying to catch him. However, it meant that she inadvertently killed his last victim, who had been abducted but not killed yet. When Olivia takes her back to her apartment to get the gun, she tearfully quotes the page title word for word before blowing her brains out. However, she herself was actually a subversion -- her only victim was an unrepentant psychopath; it was heavily implied that, had he been caught, he would've misled the cops and caused them be too late to save his last victim anyway just ForTheEvulz; and, given the time frame, her torture-caused gangrene was most likely too severe to be survivable anyway. Even the guy's own mother felt that the investigator did more good than harm.]]
%%* ''[[Series/LawAndOrderSpecialVictimsUnit Law and Order: SVU]]'''s Chester Lake.
* This was the origin of the title character in ''Series/XenaWarriorPrincess'', who started out as a villain on ''Series/HerculesTheLegendaryJourneys'' before making a HeelFaceTurn in her return arc. Xena first raised an army to protect her village from a warlord, but her brother was killed in the process. She proceeded to actively seek out possible enemies of Amphipolis and destroy them; it was not until her first encounter with Caesar that she abandoned this as an excuse.
* ''Series/{{CSINY}}'' had a soldier-wannabe who went off his meds and [[TheSchizophreniaConspiracy became paranoid]] that America "wasn't ready" for a terrorist attack. So what does he do? [[KnightTemplar He plants bombs and blows people up]], while playing CriminalMindGames with Mac and the cops.

* A patient, Curtis Ames, from ''Series/{{ER}}'', was a good man who crumbled under the loss of his right arm, the divorce of his wife, his children calling another man "dad", and losing his job. He sought to [[RevengeByProxy get even with Kovac]], who had treated him.

* ''Series/{{NCIS}}'':
** Eli David, Ziva's father, is pretty obviously this. He crossed the MoralEventHorizon, and it is obvious that he does so because of his determination to protect his people against vicious enemies.
** Jenny Shepherd is this about Rene Benoit.
* In ''Series/CriminalMinds'', the Nietzsche quote is used three times, once in the first episode, once in the hundredth episode,and once in the two hundredth episode. It's referenced in the season four finale during the finale voiceover ("How many more times will [my team] be able to look into the abyss"). However, the BAU doesn't really fit this trope, and, in the hundredth episode, [[spoiler: it's pretty clear that Hotch did the right thing]]. However, Gideon's departure from the team is due to his fear and realization that he's been staring into the abyss for too long and can no longer see humanity past it. He leaves to wander the world for a while and restore his faith in humanity.
** Interestingly enough, Gideon's reason for departing from the BAU was actually Creator/MandyPatinkin's given reason for leaving the show. When asked about it, he said that the longer he was on the show, the more and more cynical and depressed its subject made him, and he felt he had to get the hell out before it wrecked him.

** The only real difference between the way the two use people is that Red John's manipulations end in murder, whereas Jane's tend to end in arrests...but also frequently destroyed relationships, families, and psyches.
* In ''Series/{{Life On Mars|2006}}'', [[spoiler:Harry Woolf]] spends much of his career as a copper watching his nemesis become rich through illegal means while he only gets a comparatively paltry wage. To make up for this, [[spoiler:he has banks robbed and blames the crimes on his enemies, has one of the underlings of his nemesis murdered, and betrays his protégé, Gene Hunt.]]
* This happened to Jim Lahey in ''Series/TrailerParkBoys''. He was driven to great depths of depravity in his effort to save his home from the villainous machinations of Ricky and Julian. Truly, those two criminals were the shit-abyss Lahey looked into and never quite got out of.
* ''Series/VeronicaMars'' implies that Keith and Veronica's career choices are starting to take their toll on the characters's well being and sense of morality.
* In ''Series/{{Community}}'', this happens to [[SoapboxSadie Britta]], [[BewareTheNiceOnes Shirley]], and [[TheCutie Annie]] when they use [[AmbiguousDisorder Abed's]] BrutalHonesty to take down a number of [[AlphaBitch Alpha Bitches]] who humiliate other women. They end up indiscriminately pointing out the flaws of everyone, until it reaches the point [[MetaGuy Abed]] invokes HoistByHisOwnPetard by giving the former {{Alpha Bitch}}es the perfect insults to use on them.
* ''Series/TrueBlood'':
** Antonia was a witch who was raped and murdered by vampires in the middle ages. When she comes back as a spirit, the next logical step is to attempt genocide against the entire vampire race, attacking and imprisoning everyone that stands in her way.
** In season 6, [[https://www.youtube.com/watch?v=Uv_E0sYqzPA Bill Compton asks his protegee/surrogate daughter Jessica to keep him grounded]] after he drinks the blood of Lilith and gains new abilities like being able to move objects with his mind, or survive being staked. To explain why he needs Jessica to ground him, he cites the story of General Sherman from the Civil War: a man who hated war, but who became more and more ruthless as he gained power and rose in rank, to the point of his army burning down towns on their march to the sea.

* In ''[[Series/SpartacusBloodAndSand Spartacus War Of The Damned]]'': A number of the rebels are showing signs of this, as seen when they slaughter innocent civillians including children. Spartacus wants them to be better than the Romans, but is unable to keep them in line. Gannicus is aware of what they are becoming, but seems to have resigned himself to the inevitability of it.
* ''Series/{{Arrow}}'' gives us [[ComicBook/{{Huntress}} Helena Bertinelli]], who is initially shown as a WoobieDestroyerOfWorlds type because her father killed her fiancee. Naturally, she wants revenge on her father. However, her plot involves [[MoralEventHorizon killing and maiming multiple innocent people.]] LikeFatherLikeSon at its worst.
** The Dark Archer's character arc is precisely this. It just happened long enough ago that he is virtually indistinguishable from a KnightTemplar.
** As expected from a character inspired by [[{{Franchise/Batman}} the epitome of avoidance of this trope]], the titular Arrow struggles with this in some form or another every season.
* The lead character of ''Series/{{Hannibal}}'' -- who is not the eponymous Lecter but rather FBI [[TheProfiler profiler]] Will Graham -- is CursedWithAwesome[[AwesomenessByAnalysis ness By Analysis]]. He inspects the crime scenes of serial killers and reconstructs means, motive and pathology from them - almost literally reliving the crime as it was committed. Needless to say, what he finds in the minds of those killers is pure NightmareFuel, and basically every character in the cast cautions Graham's FBI superior and Will himself about the possibility of this trope. And then we add the fact that the eponymous Lecter is TheCorrupter, who has every reason to push Graham into that abyss...
* ''Series/{{Highlander}}'' The Dark Quickening is this. It's what happens when a good immortal takes in to much evil from the others they defeat and corrupts them.
* ''Series/BabylonFive'' has this in spades:
** The first and most obvious example is the Narn. They used to be a peaceful and technologically primitive race before the Centauri conquered their planet and enslaved them. After decades of fight, the Narn managed to force the Centauri out... And promptly started using the technology they stole from the Centauri to conquer their neighbours while they prepared their revenge against the Centauri.
** [[{{Irony}} Ironically]], the Centauri themselves (whose RPG rulebook even starts with Nietzsche's trope-naming quote). Before first contact with the Xon, the other sentient race of their own homeworld, they were peaceful artists who had even rejected the very concept of war. Then a naval expedition reached the Xon lands, causing the Xon to find out about them and attack the Centauri, killing and enslaving many of them. By the end of the war, that also included a brief alien invasion from the Shroggen, no Xon was alive, and the Centauri were a fledgling empire ruled by a DeadlyDecadentCourt and bent on expansion to get even with the Shroggen and protect other races. With time, they forgot their motivation.
** The Minbari in general and their Warrior Caste in particular. After the last war against [[AbusivePrecursors the Shadows]] they spent a thousand years to prepare for the next, and just as it was coming [[PoorCommunicationKills a screwed-up first contact with Earth caused the death of their political and religious leader]], prompting them to start a genocidal war in spite of the humans trying to surrender multiple times. They stopped and surrendered right after destroying the last of Earth's military and a few minutes before actually enacting the genocide, thanks to [[spoiler: finding out evidence that Minbari souls are reincarnating in humans]], but, partly because the motivation was kept from the public, it takes a while for the Warrior Caste to stop behaving like everyone is beneath them (the first time we see a Minbari warship in the series, it repeats the same mistake that caused the war. Thankfully Delenn was there to explain that custom).
** The humans themselves. After the devastation of the [[HopelessWar Earth-Minbari War]], in which their allies abandoned them out of fear and the only help they received was weapons sold to them by the Narn, many humans, especially in the government, felt they had to do ''anything'' to prevent this from happening again, including [[spoiler: killing the president of Earth Alliance in a fake accident and [[DealWithTheDevil allying with the Shadows]]]].
** The episode "Infection" went into the history of the people of Ikarra 7, who were repeatedly invaded by aliens, and in a desperate attempt to throw off the invasion, built a dozen war machines to combat them. Unfortunately, the machines were programmed by religious fanatics who had a ''very'' narrow definition of "pure Ikarran" (the only people they would accept commands from--and ''no one'' met the definition), and the war machines destroyed ''everything''. As Sinclair put it to the last such machine:
--> '''Sinclair:''' You and the rest--you forgot the first rule of the fanatic: when you become obsessed with the enemy, you ''become the enemy''!
** In the episode "Dust to Dust", Ivanova ''almost'' uses the station's defense grid to shoot down recurring nemesis [[SmugSnake Bester]]'s fighter in what she would have [[MakeItLookLikeAnAccident attempted to frame as an accident]]. Sheridan arrives in C&C in time to stop her, then admonishes her:
--> '''Sheridan:''' Fight them without ''becoming'' them.
** Sheridan himself falls victim to this, when he suddenly encounters a survivor of the accident that he'd previously believed his wife'd died in. In a mad hope that if this man, Morden, is alive, then she might be as well, Sheridan unlawfully locks him up and interrogates at length, dismissing his own crew's objections. Furious by Morden's refusal to cooperate and suspecting him of lying [[spoiler:(he ''is'' lying)]], Sheridan threatens to keep him locked up indefenitely and even ''torture'' him.
* In the ''Series/MastersOfHorror'' episode "Incident On and Off a Mountain Road", Ellen eerily takes on many of the villain Moonface's mannerisms at the end. She gives [[spoiler:her dead husband]] the same treatment Moonface gave to his victims and kills [[spoiler:Moonface's insane captive Buddy]] to tie up all loose ends.
* In ''Series/{{Castle}}'', it's revealed that Detective Kate Beckett's mother was murdered as the result of a lengthy chain of events that resulted from a trio of cops who, cynical about the justice system's ability to effectively deal with the mob, eventually rogue in order to bring them down. In her efforts to expose the people behind her mother's death, it gradually becomes clear that Beckett is beginning to take on several similarities to these cops, including going rogue at times. [[spoiler: It's ultimately subverted; she ends up having an epiphany in which she realises she's in love with Castle, is throwing her life away on revenge and decides to step back and focus on building a life with him rather than spiral into self-destructive obsession.]]
* In ''Series/TheManInTheHighCastle'' this is a general theme for the resistance. While doesn't apply to ''all'' of them all the time, in order to keep their fight up they have had to make moral sacrifices. The most prominent example is [[spoiler:George Dixon. Dixon]] wants nothing more than to see America free from Nazi rule. [[spoiler:to achieve that end he spies on people using the Nazi's own surveillance lines, even other people he is suppose to be on the same side with. He is willing to kill any Nazi for his cause, even the civilians. He plans to have the senior most Nazi officer killed by his own people by revealing that his son has an incurable genetic illness, even though that will kill an innocent child in the process. Juliana says he is just as evil as the Nazis and his response is that they must be eviler to win and then justifies his actions by arguing that the boy he would kill is sick anyway. He dies in a Nazi uniform, a disguise he was wearing at the time, when Juliana refuses to let him do this.]]

* In ''Series/TheEscapeArtist'', Will's position forces him to defend criminals who may well be horribly unpleasant sociopaths. [[spoiler:By the end, he ends up murdering Foyle and successfully getting ''himself'' out of a murder charge, although in a variation he's able to move on]].
* In the ''Series/StarTrek'' episode "The Savage Curtain," The Excalbians notice that both the "good" team and the "evil" team used the same tactics. Kirk explained it was ''the reason'' that they fought, the Enterprise crew was threatened if the good team didn't fight while the bad team was offered "power" if they won.
* Several episodes of Series/StarTrekDeepSpaceNine had various members of the crew struggling with exactly this

** Sheridan himself fell victim to this, when he suddenly encounters a survivor of the accident that he'd previously believed his wife'd died in. In a mad hope that if this man, Morden, is alive, then she might be as well, Sheridan unlawfully locks him up and subjects to a lengthy interrogation, dismissing his own crew's objections.

** In his youth, Robert Baratheon led a rebellion to depose the cruel and paranoid Mad King Aerys II, but as king he resorts to increasingly unsettling means to keep his own dynasty on the throne, though, unlike Aerys he does realize that he's going too far, and tries to call off his hit on Daenerys on his deathbed.

** In his youth, Robert Baratheon led a rebellion to depose the cruel and paranoid Mad King Aerys II, but as king he resorts to increasingly unsettling means to keep his own dynasty on the throne.

** Dark Willow started out wanting revenge against Warren for what her did to her girlfriend, Tara, then moved onto his friends because of guilt by association too, and soon escalated into CardCarryingVillain territory, until she eventually [[OmnicidalManiac tried to destroy the world]]. Willow was brought back from the abyss by [[ThePowerOfLove Xander]].