Most of the Argent family from Teen Wolf. 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 Knight Templar, 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 Manipulative Bastard.
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.
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 Card-Carrying Villain territory, until she eventually tried to destroy the world. Willow was brought back from the abyss by 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 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.
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.
Doctor Who: The Technical Pacifist 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 Sliding Scale of Idealism vs. Cynicism quite frequently. In one case, he was attacked by creatures who wanted to steal his immortality. 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 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 "Horror of Fang Rock" may be this in their long war with the Sontarans.
Here's a capper: In the Doctor Who (Titan) 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 Twilight Zone "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 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 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 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 Death Seeker 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 kills Amy Pond (not that one) because he can't trust a monster not to kill again, complete with a 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 demon-blood-fueled powers also lets him save the hosts when exorcising demons, his obsession with gaining the power to kill Lilith 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.
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 kill defense attorney Ellen Wolf, and 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 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 both Marina and Renzo, the former shooting her brother's murderer and the latter burying the main villain of the season... alive.
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 taking Sylar's power in order to save the world caused him to also gain Sylar's hunger.
One season 9 episode of Law & Order: Special Victims Unit had a serial killer found dead in the same manner as his victims. 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 For the Evulz; 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.
This was the origin of the title character in Xena: Warrior Princess, who started out as a villain on Hercules: The Legendary Journeys before making a Heel–Face Turn 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.
In season seven, Tony displays this trope. To the point where he actively kills innocent people, and the FBI agents trying to find him, all so he can have revenge.
Jack Bauer takes this trope Up to Eleven in the second half of the eighth season, when he gets his hands on a murderer. 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 re-imagined Battlestar Galactica saw the Resistance on New Caprica using suicide bombers against the Cylon occupation force. Colonel Tigh gives us this quote. He's being partly sarcastic, though.
"Which side are we on? We're on the side of the demons, Chief. We're evil men in the gardens of paradise, sent by the forces of death to spread devastation and destruction wherever we go. I'm surprised you didn't know that."
A patient, Curtis Ames, from 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 get even with Kovac, who had treated him.
Dark Shadows has Reverend Trask, a self-styled witch hunter who had undoubtedly killed many innocent women. As a ghost, he's finally talked into a Heel–Face Turn thanks to the opportunity to finally destroy a real witch.
Lost: considering who the series's ultimate Big Bad is, this can be inferred as the reason for much of the Others's villainous behavior.
Ben Linus especially. He only kills those he sees as trying to hurt the island, dedicating pretty much his entire life to protecting it for Jacob. To quote him: "When I'm at war, I'll do what I need to do to win, but I will not kill innocent people."
Eli David, Ziva's father, is pretty obviously this. He crossed the Moral Event Horizon, 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 Criminal Minds, 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, 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 Mandy Patinkin'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.
Patrick Jane is so much this when it comes to Red John. The show even goes so far as to hang a lampshade on it in the season 3 episode "Red Moon:"
Jane: I have spent enough time with that creep. Staring into the abyss—you know, it's not healthy.
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 Life On Mars, 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, 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 Trailer Park Boys. 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.
Veronica Mars implies that Keith and Veronica's career choices are starting to take their toll on the characters's well being and sense of morality.
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, 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.
King Uther. He lashes out at the death of Ygraine due to the magic used to conceive Arthur, and launches into the Great Purge, killing everyone in Camelot even suspected of using magic, and forever banning magic in the kingdom. Except he invented most of the 'monsters' in his grief.
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.
Likewise, Morgana started off as a heroine in series 1-2, someone who would defy the king to help, for example, Merlin's village fight off the bandits attacking it, or demand of Arthur that he save Gwen, a mere servant in Uther's eyes. As time goes on, and she develops magical powers, and Uther kills others who she knows who he suspected of sorcery (notably Gwen's father and the druids who take her in), her attitude to Uther becomes more and more poisoned, until she attempts to kill him twice. The first time, in a subversion to the trope, she performs a Heel–Face Turn when she sees that he is truly sorry for what he has done. On the second occasion, after Uther has continued as he was, she becomes fully committed to killing him, and by the end of her training with Morgause, she is willing to manipulate and kill anyone who stands in the way of her destroying Uther, and everyone related to him.
His mentor Cara Stanton was even more aware of it, and even tells him such when he joins the CIA: "We don't walk in darkness, we are the darkness."
FBI Special Agent Donnelly is an interesting example. Being a Sympathetic Inspector Antagonist, 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. 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.
Space: Above and Beyond gives us Col. Ray Butts, who apparently was born mean and became meaner from being a Marine lifer—he's racist against InVitros, picks pointless fights with the Wildcards, antagonizes McQueen by taking the squad away from him for a mission he won't explain to anyone, and changes mission parameters mid-mission, again without any sort of explanation. It gets to the point where the squad briefly wonders if he might have killed his previous squad members when they fight a dead marine's body on the planet. the squad was actually killed by chigs when they wanted to wait for reinforcements—causing Butts to leave them in disgust to do the mission on his own
In 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.
The lead character of Hannibal — who is not the eponymous Lecter but rather FBI profiler Will Graham — is Cursed with Awesomeness 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 Nightmare Fuel, 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 The Corrupter, who has every reason to push Graham into that abyss...
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.
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.
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 Deadly Decadent Court 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 the Shadows they spent a thousand years to prepare for the next, and just as it was coming 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 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 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 killing the president of Earth Alliance in a fake accident and 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 Bester's fighter in what she would have 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.
In the Masters of Horror episode "Incident On and Off a Mountain Road", Ellen eerily takes on many of the villain Moonface's mannerisms at the end. She gives her dead husband the same treatment Moonface gave to his victims and kills Moonface's insane captive Buddy to tie up all loose ends.
In 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. 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 The Man in the High Castle 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 George Dixon. Dixon wants nothing more than to see America free from Nazi rule. 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 the fifth season of The Shield, Jon Kavanaugh starts out as a well-meaning (if self-righteous) Internal Affairs officer investigating the corrupt Strike Team, especially Vic Mackey. But as the season progresses, Kavanaugh's quest to take Vic down becomes increasingly personal, desperate, and obsessive. In Season 6, Kavanaugh finally becomes a dirty cop himself, planting evidence and coercing false testimony against Vic, which leads to his own downfall. As Kavanaugh puts it in his final episode, "I framed a guilty man."
In The Escape Artist, Will's position forces him to defend criminals who may well be horribly unpleasant sociopaths. 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 Star Trek 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.
The two-part episode *Homefront/Paradise Lost* features 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 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 in their role.