Avoided in Higurashi. Shion goes insane and embarks on an attempt at revenge after the boy she loves, Satoshi, disappears. A lot of people are captured, killed or both, including her twin sister, her sadistic grandmother, and the village headman, who are all part of the village mafia. Then Shion goes after Satoshi's little sister, Satoko, because she's mad that Satoko's dependence on Satoshi wore him out. Shion captures Satoko and tortures her to death. Then she remembers that Satoshi's last request was that she care for Satoko for him. Shion was already crazy, but now she loses any pretense of acting for anything besides her own dark pleasure.
To add to the issue it's revealed their deaths were in vain. None of them had to do with Satoshi's disappearance, and if anything they all liked him.
Light Yagami begins using the supernatural notebook to rid society of criminals, but soon his black list expands to include anyone who stands in his way for any reason, starting with the FBI. Along the way, he coolly manipulates the feelings of both people and shinigami. Repeatedly stating that he plans to become the god of the new world he is trying to create doesn't help matters, either.
For that matter, Teru Mikami uses the notebook to eliminate minor and reformed criminals. Eventually, he declares he will start executing people who are just lazy or do not contribute to society. Light has to work to stop him, not because he doesn't want the book to be used like that, but because he believes it's too early to start doing that. Meaning that Light inevitably plans on going down that pathway, once he's rid the world of the rest of the more undesirable people in the world.
Shu Ohma in Guilty Crown after Hare dies. Now regards all the 'F-Rank' Void users as worthless cannon fodder, sending them on suicidal missions to get supplies (including from a sunken ship, even the idiot Souta), eventually; after learning that breaking a void kills someone, in addition to a Trauma Conga Line, he starts on a path to The Atoner.
In Code Geass, Lelouch Lamperouge wanted to destroy the Empire of Britannia (despite being an ex-Imperial Prince) and started to organize La Résistance. However, as time passes, he becomes more hardened and crazy, going as far as massacring children and unarmed people for possessing Geass powers and being a part of a Britannian-sponsored cult... which trained the children in it to use their Geass to make a guy murder his own allies. Let us not forget Rolo, THE Tyke Bomb of the series, was raised there too.
And of course, we have The Emperor and V.V, who after suffering immense loss as children, do the exact same thing fifty years later, even though they said they will eliminate lies. Not to mention V.V. is to blame for ordering the brainwashing and training of Rolo and the kids, and also lying to and betraying Charles himself in addition to instigating some of the worst twists in the series...
In general, everyone in Geass flew off slippery slopes.
Lelouch and Suzaku are the extreme cases. As well as the others named above. Ironically, Euphemia, who actually goes on a rampage, killing Japanese people, only did so because she was under the power of Geass and had no choice.
If you truly think about it the entire show is about Lelouch going from the fettered to the unfettered. He slowly slips into darkness as the series progresses and using more brutal tactics to achieve his ends. This happens because his situations demanded it and his own personal losses take a toll on him like killing Shirley's dad, to having to geass his best friend, to having to kill his sister, to having Shirley die in his arms, to having his main goals snuffed out because they either died or became lies, to having the army he founded betray him, everything about Lelouch's life sucks. Compare his methods at the beginning villeta being geassed to give up her knightmare instead of killing her and taking it to having legions give up their freedom at the end. Lelouch goes off the deep end but spends the better part of two seasons to get there.
There's also Nina. Who has one of these when her idol Euphemia is killed.
In Mobile Suit Gundam 00, the Trinity group shows up and starts actually destroying military bases and arms manufacturers. Most of the battles up until this point had hundreds of civilian casualties, with one battle threatening to basically screw the entire world with nuclear radiation. Through destroying military installations instead of waiting for war to start, Trinity is preventing these needless deaths. However, the "it's not right to attack before you're attacked" excuse is played, Trinity is painted as villainous when it's actually clearly good... and it suddenly starts blowing up random buildings for no reason. Let's also point out that the villains out to cause perpetual war for personal profit are never portrayed in nearly such a negative light.
While Trinity's actions may have been jumping of the slippery slope they were hardly 'clearly good' in the first place- Gundam 00 is full of Grey and Gray Morality and the point is that no one side is clearly good or clearly evil. The Trinity's actions were simply causing more needless deaths by murdering civilians in arms factories and blowing up military bases which weren't attacking anyone, simply defending their homeland. Trinity's actions are a classic example of jumping of the slippery slope by quickly resorting to overly extreme methods, and are hardly unambiguously good ones being wrongly portrayed as evil.
It's the difference between shooting the gun out of someone's hands and shooting their hands off so they can't wield a gun in the first place.
Depending on your viewpoint, Celestial Being itself was doing a lesser version of this before Trinity even showed up. They certainly had no compunctions about blowing up a training facility for Super Soldier children, and all the civilians inside, as a revenge operation.
Allelujah certainly had his compunctions.
As did the rest of Celestial Being. Halleluah, Allelujah's dark half, enjoyed it and egged him on.
It must be noted that Celestial Being actively tried to avoid civilian casualties, such as Lockon shooting to disable enemy suits when possible, or Setsuna taking off their limbs but not gutting them or cutting them in half. Celestial Being attacked only areas of active combat, and repeatedly tried to prevent situations that would increase civilian casualties, such as stopping a terrorist attack on a nuclear waste dump. Trinity, by contrast, repeatedly attacked civilians targets (including simple factories) and expressed neither remorse, guilt, or sadness for it, and then in a childish fit attacked a WEDDING. That's why the Trinities are portrayed as much farther down the slope.
Yet it must be pointed out that Gundam 00 was heavy on the All In The Manual aspect and the side documents explained that the Trinities resorting to extreme measures and having sociopathic natures were not of their own intentions, but rather as a result of their development. Because their role was to make Celestial Being look bad, and then die immediately aftewords, their resorting to harsh measures was simply to cause a public relations nightmare with the group, and to ensure that Celestial Being would not bother helping the Trinity siblings in their hour of need (Setsuna did not intentionally save Nena, but rather was attempting to stop Ali al-Saachez from using a Gundam, especially since they used the Trinities' fight for survival as battle strategy for opponents). That's Fridge Horror in a nutshell.
The Big Bad of Air Gear jumped long before the series began. At one point he contemplated concepts such as mercy and forgiveness, then he put on a pair of Jade Coloured Glasses and became utterly ruthless.
The last two episodes of the first generation in Mobile Suit Gundam AGE shows Flit Asuno taking the dark descent to becoming a revenge-obsessed Earth Federation fanatic. His refusal to accept the UE as human beings and his Unstoppable Rage from seeing Yurin die only matters worse for him. As an adult, he leads a task force that purges the Federation government of any and all Vagan sympathizers, accusing them of collaboration and having them executed. When it's pointed out that this action will kill any chance of a peaceful resolution to the conflict, Flit merely says "Yes, I know. That was my intention from the beginning". Honestly, the man is getting dangerously close to the Moral Event Horizon here, if he hasn't crossed it already. Then he raises his grandson as a Child Soldier to take his place, is outraged when said grandson decides committing Vagan genocide would be bad, and advocates to have a captured base destroyed with an untested superweapon despite the presence of enemy civilians and Feddie prisoners.
Zeheart Galette has crossed it in the third generation by putting his full faith in Ezelcant's true Social Darwinist agenda, despite the fact that it involves killing Vagan citizens and Ezelcant himself admitting that he was insane. The next episode, he's fully willing to open the EXA-DB, a database of all weaponry from previous, devastating wars, despite warnings that it is basically a Pandora's Box. Before this, he was a committed Anti-Villain who made a point of remembering that The Dead Have Names, still acknowledged that Asemu had been a friend, and said that he was fighting to return his people to the "Eden" of the Earth Sphere.
Also narrowly averted in the 2003 anime version when Ed almost uses the prisoners trapped in Lab 5 to create a philosopher's stone he's been searching for.
Fushigi Boshi No Futago Hime: Fine and Rein find out that Mirlo is in an Arranged Marriage with a rather undesirable dimwit, and are out to break it up. Reviewer Al1701 pointed out that this action seems short-sighted, since the deal for the marriage is in exchange for dimwit's father repairing the Waterdrop Kingdom's cloudmaker. That is, until the whole Arranged Marriage turns out to be a big ruse by the Moon Kingdom chancellor. Doesn't stop this from being one of the best eps of the whole series.
In Berserk, no one ever really addresses Griffith's actions, since they're so amazed at how far he's willing to go in order to achieve his dream of having his own kingdom. Even if those actions may have included whoring himself out to an old pedophile to raise funds for his army, or planning assassinations to eliminate opponents or kidnapping kids and using them as leverage against other opponents. However, the audience is shown this from a more sympathetic light, especially so after his one year imprisonment and torture which destroyed any chance of him achieving his dream since he speaks of his dream so nobly and pure. And then the Eclipse happened. The audience lost all sympathy at that point.
The Killing Joke is all about The Joker trying to break Commissioner Gordon. He fails. Ironically in virtually every adaptation of Joker's origin story this trope is the main reason he... um, well... is the way he is.
Averted sometimes when he doesn't kill (for various reasons, mainly when having to team with the majority of the protagonists), and averted in his MAX series where to date things have been almost entirely on the cynical side and... Then again, arguably he's not treated that nicely there either, it's just that his targets are apparently much, much worse, and the reason that he keeps a select few alive? Well...
Garth Ennis admitted that his sheer anger regarding human trafficking and sex slavery led to the infamous arc "The Slavers," which has Frank Castle graphically disembowel a slaver, throw his sister face first into a window repeatedly until the shatterproof window breaks off from the frame, and then set their father on fire... even after Frank admits that this won't make a big difference and that he's just going after their group, and at the end his inability to really help the victims.
In the miniseries Born it's pretty much stated the Punisher is a sociopath who's addicted to killing, and that if it wasn't criminals it would just be someone else. It is also sort of ambiguous as to whether he sold his soul to some sort of demonic entity, or was just going increasingly insane.
It's arguable how far down the slope he already was, but the arc Superman: Ending Battle is this for Manchester Black. Initially, Black was a Type IV or V antihero who managed to just hover on the line between Chaotic Neutral and Chaotic Evil, but, after he learns Superman's secret identity, he leaps across the Moral Event Horizon; telepathically takes control of the supervillains (literally, all the supervillains) to attack anyone who's ever had any contact with Clark Kent, mind rapes the rest of the Elite (the only people who give a damn about him), and finally tortures and pretends to kill Lois Lane, all as part of a massive Thanatos Gambit to destroy Superman by goading him into breaking the One Rulepointlessly. When Superman refuses, Black suffers a major Heel Realization and telekinetically blows his own brains out.
Hal Jordan, who got so pissed off that the Guardians forbid him to use his ring to temporarily recreate Coast City that he flew to Oa, maiming numerous other Green Lanterns in the way and stealing their rings (how they survived the vacuum of space is unknown), killed Kilowog, killed Sinestro, absorbed the whole power of the main battery (destroying it in the process), became the villain Parallax and then tried to destroy the universe in order to recreate it "the right way" actually, successfully destroyed the universe, but then the heroes hijacked his attempt to recreate it to make an acceptably similar replacement. Of course, it was later retconned as he being possessed by the fear entity Parallax, but still...
Notably averted by Alan Grant's DC Universe character of Anarky: where originally the character was scripted to be willing to murder in pursuance of his anarchic philosophy, as written he upholds the same moral standards as Batman, which makes for some nice Not So Different interactions.
In Superman Annual 3 in the Armageddon 2001 crossover, Superman declared war on all nuclear weapons. At first, he just took away all nuclear weapons. Then, he started to steal from rich countries to give to the poor countries. Over the course of ten years, he became more intense and actually started sinking submarines that has nuclear weapons on them. When people started to die (accidentally), everyone started to get worried that Superman has gone too far. So, Batman decided that he had to kill Superman with the kryptonite ring.
Played with when Scans Daily showed a panel with Robin and the Spoiler foiling a convenience store robbery and Stephanie taking a soda. Tim assumes she's going to pay for it, while she explains that she's earned it as they saved the whole store. She grudgingly puts down some money when Tim explains that once you start like that, you soon bend all the rules.
SD: "It just starts with stealing a soda after saving the store... next thing you know, you're hacking up people and putting them in your freezer!"
In Star Wars: Legacy Emperor Roan Fel is willing to use Sith powers to get his Empire back. He finally slips to the dark side when he plans to use a bio weapon on Coruscant to wipe out the Sith, unbeknown to him they are immune to it, but it will kill his allies and billions of lives still on the planet.
Dealt with in a two-issue arc of New X-Men involving Prodigy, a student at the Xavier Institute with the ability to absorb the knowledge of anyone in the immediate vicinity, but only as long as they're in the immediate vicinity. When Emma Frost discovers that he's subconsciously put a block in his mind that prevents him from retaining the knowledge permanently, David asks Emma to remove the block, figuring he'll be able to do so much good for the world that way. The story then fast-forwards a few months, and David is already head of his own Mega Corp. that has developed cures for cancer and AIDS. How did he achieve this breakthrough? He killed his friend, the healer Elixir, and harvested his organs. Okay, that's certainly bad, but we're still at the point where it can be argued to be worth it. The next issue fast-forwards again, to twenty years later, and David is now the President, and has united most of the world's countries into a utopian One World Government. And he's also planning the genocide of the Chinese because they refused to join. Okay, now he has to die. Fortunately the whole thing turns out to be an illusion, courtesy of Dani Moonstar, in an effort to convince him to leave the block in.
Another X-Men example in one of X-23's creators, Dr. Xander Rice: He forces Dr. Kinney to carry Laura's fetus to term by refusing to allow her to search for another host (so it's do it, or let the embryo die), shoves the girl into a radiation chamber at the age of seven to forcibly jump-start her healing factor by nearly killing her with radiation poisoning (when it's implied he could have gotten the same results with a "safer" dosage. Or just allowed her to manifest naturally), denies her anasthesia when he surgically removes her claws several days later to coat them in adamantium, uses Cold-Blooded Torture to condition her to the trigger scent, puts her in the "care" of an Ax-Crazy psychopath, treats her as an animal and subjects her to years of physical and emotional abuse, tries to kill her at one point by abandoning her on a mission, and oh yeah, sends her to kill the man who practically raised him from a toddler, and the man's wife and son (who is actually his son from an affair with the woman). Rice doesn't so much as jump off the slippery slope as take to it in a rocket sled. And then keep on going.
Jean-Paul Valley's tenure as Batman was exactly this. When given the mantle of the bat by the injured Bruce Wayne, he started out simple and was willing to follow rules. However, after the Scarecrow doused him with Fear Gas and the System kicked in, Jean-Paul's morality started to waver. He came close to viciously murdering villains such as The Joker, Tally Man, and Mr. Freeze, but after letting Abbatoir die by falling into a vat of molten steel, letting his captive die, he decided to embrace his more vicious and brutal side once and for all, leading to Bruce to come back and take the mantle from him by force.
In the Chick Tract"Fairy Tales," a young boy named Harry has a shocking revelation that immediately causes him to go on a violent rampage and become a mass murderer, ultimately winding up next to Osama Bin Laden on the FBI's most wanted list. What was this world-shattering revelation, you ask? He realized that there's no reason for morality when he was told that... Santa Claus, the Easter Bunny, and the Tooth Fairy aren't real!
Ho-oh from Poké Wars wants Pokémon-kind to live in a utopia and he limits his targets to humans only and tries to limit the damage he causes. He soon starts engaging in things like ordering wanton killing of Pokémon contrary to his ideals, utterly ruining the environment and not giving a damn about it, and generally being a filthy hypocrite.
Summer Days And Evening Flames: Sergeant Sherry's logic for starting a gang war is sound, in a twisted sense, as it eliminated all the major players in the Farrington crime circuit. However, she somersaults off the ledge when she's about to be arrested for the indirect murder of dozens of city guards, where she shows little remorse and violently escapes. Sherry made a perfect landing when she pulls all of her connections to get Iron removed from his post and threatens him and his family if he pursues her.
Mass Effect Human Revolution's version of Toombs. It was one thing when he was hunting the Illuminati scientists who conducted unethical experiments on him, but he lost any moral high ground when he started taking hostages with intent to kill in order to force out into the open the one who rescued the final scientist.
In the The Dresden Fillies story False Masks, the plot is that the Order Triune, an ancient society have mistaken Harry Dresden to be their ancient foe, resurrected after 1200 years, and try to kill him. They attempt to poison him, and then kidnap the ponies close to him, like Twilight. When their coverts efforts fail, they take a direct approach. Summon HE WHO WALKS BEHIND (TWICE) to kill him, ignoring all the ponies around him that could have also died. And when that fails, they were ready go through the Mane 6 and the princesses to kill him, and would have done so, had they not been sacrificed themselves to summon a demon. And in turns out that Obsidian never was resurrected, so they had committed all these crimes for nothing.
Yoda of Star Wars warns of the danger of The Dark Side, giving a slippery slope argument on how it works: "Fear leads to anger, anger leads to hate, and hate... leads to suffering." However it is exceedingly rare for somebody to not simply go to hate and Dark Side in the EU. ...although, in truth, that statement of fear leading to hate isn't entirely accurate; fear is itself a symptom of greed. Self-interest (for people to value things rather than other people) is the root of all hatred; the desire for others to suffer, which is, in truth, the definition of evil.
Anakin Skywalker from the prequels. Specifically, note how quickly he goes from agonizing over his role in Mace Windu's death to killing younglings without a problem. Anakin finally slips so far, his own wife, Padme Amidala, loses the will to live and eventually dies, and Obi-Wan Kenobi is forced to duel him, ending gruesomely. In order to keep Anakin alive, Emperor Palpatine, a.k.a. Darth Sidious, subjects him to painful body reconstruction. The final push to the Dark Side comes from Palpatine himself- when Anakin asks if Padme is still alive, the Emperor tells him in his anger, he killed her. In pure disbelief of this, Anakin's true power flares up, and he screams out in despair, fully overtaken with pain and hate, completing his transformation into Darth Vader. What was especially loathsome of him was that, rather than desiring everyone's well-being, his honest desire for good was corrupted into infatuation and affection (he wanted to be together with her, alive, rather than desiring what's best for everyone); a desire to be with her for the sake of his pride; to feel good about himself for being able to keep the people he knows personally from dying (although, sadly for him, Palpatine never had any concern for his wife to begin with).
Fortunately, it was not to last. By the sixth film, it becomes evident Vader shows signs of regret when Luke challenges his motives, but Vader dismisses them and says it is too late for him- no one can leave the Dark Side. It isn't until Palpatine repeatedly strikes down Luke with Force lightning that Vader finally comes to his senses and reverts back to Anakin Skywalker, slaying the Sith Lord, but at a very costly price.
Not very well fleshed out in the movies, but thefluff explains Anakin's motives as him being a Knight Templar. He's tired of people dying, the war, sees himself as the hero (since he's been told he is The Chosen One since childhood and thus can do no wrong) and is stupidly loyal to the Republic. All that makes him childlishly easy to manipulate by Palpatine. Combined with the corrosive effects of the dark side it makes baffling lapses of judgement come easy to him.
The character of Amanda Young in the Saw series makes Jigsaw look downright merciful by the third movie. Of course, this was the fault of Jigsaw himself, who made her a murderer in an attempt to "help her", much to his shame when he realizes this. It didn't help any that Jigsaw's other protégé, Mark Hoffman, was already far down the slope, tugging on her leg at the time.
Magnum Force has Dirty Harry dealing with cops who have been executing guilty criminals who escaped justice due to technicalities. When he refuses an offer to join them, they try to murder Harry, thus proving they didn't have a complex or unorthodox sense of justice after all, they just like killing.
The real Moral Event Horizon was before that when they murdered Harry's unstable friend, Officer Charlie McCoy, simply for being in the wrong place at the wrong time.
Crimson Tide is often noted as quite admirably morally complex for a Jerry Bruckheimer film, with Gene Hackman's character given quite a bit of sympathy in wanting to launch the missiles. At least until the ending, when he makes a thinly veiled racist comment to Denzel Washington, which Washington promptly reverses on him.
There's also the fact that Gene's character gets increasingly loud as the movie continues, yelling and screaming at people, while Denzel remains calm and logical, which serves to show you just who's right.
However, the Navy board of inquiry at the end of the film sums it up as "You were both right, and you were both wrong."
Potentially justified, the kid was setting him up to get shot, and they were at war.
In Super, Rainn Wilson's character decides to fight crime. This begins with trying to stop drug dealers, but turns into him brutally beating people with a monkey wrench (for cutting in line at the movie theatre).
A German Film Stahlnetz: PSI begins with two brothers kidnapping a rich girl actually she is not... for ransom. They reason a girl is a Spoiled Brat anyway and a few days in captivity won't hurt - and for her family, a million is only a pocket money, so it's not really bad. But then the younger brother decides that it is better to leave the girl to die, and when the other brother objects, beats him up and locks him together with the girl to die.
In Jumper, the Paladins believe that Jumpers will do this eventually, which is why they get targeted for preemptive killing.
In The Star Chamber, the turning point comes, if not before, when Hardin and the rest go vigilante in order to pursue justice as they see it.
In Ghostbusters, EPA agent Walter Peck had a point that the Ghostbusters' operations needed some inspection, especially considering Egon was getting worried about the abnormal amount of spectral energy they are containing in their protection grid. However, when Peck later vindictively barges into the business with a court order and imperiously orders the grid shutdown despite the warnings of the Busters and the reservations of the accompanying utility worker, he is shown to be totally unreasonable and completely in the wrong. The fact that Peck then orders the Busters arrested for an explosion he himself clearly was responsible for makes him truly despicable.
Discussed between the president and Beast in X-Men: The Last Stand, when they debate weaponizing the cure as a safeguard against dangerous mutants. The president tries to rationalize it as a extreme circumstance, to which Beast responds by pointing out how quickly such a justification can snowball before resigning. The president ends up doing just that when Magneto's mutant army shows up, but by the end he's climbed back up and rehired Beast to help smooth things out again.
For all its faults, X-Men Origins: Wolverine's opening montage of the many wars the US has been in did show Victor's decline from a soldier doing his job to a ruthless killer quite nicely without being really obvious about it.
Young Erik has grown increasingly amoral since his actions in First Class. Mystique isn't far off, either. The two split right before Dallas.
Trask Industries' Sentinels originally targeted mutants. But as time passed, their programming came to include regular humans whose offspring would be mutants and eventually, anyone sympathetic to mutants who stood in their way. This ultimately brought about the Bad Future where the worst of humanity lord over the remnants of civilization, using the latest Sentinels to finish off the mutants once and for all.
Oh dear lord Jacen in Legacy of the Force. In the first book he has a vision that the galaxy will fall into chaos and he will end up killing his mentor Luke Skywalker unless he listens to the Villain of the books, and is forced to kill one of his allies who refuses to listen to Jacen's reasoning. Cut to book two when he tortures a prisoner because she knows about a plot to kill his parents and accidentally kills her. Cut then to book three where it is he who is trying to kill his parents because "My parents are terrorist scum, and that is why I have to show no mercy towards them." This might be a clever showcasing exactly how "Falling to the Dark Side" works - turning the most justifiable cause into For the Evulz-Obviously Evil.
In Damon Knight's short story "The Analogues", a scientist invents a procedure to create a "better conscience" in the form of hallucinations that prevent you from committing crimes. This raises a lot of questions about the morality of removing free choice, but then it turns out the scientist plans to use it to take over the world, and has already used it on the protagonist to prevent him from stopping the plot.
In The Dresden Files, the White Council is extremely strict with their 7 Laws of Wizardry. First violation means an instant beheading unless a wizard on the Council bets their life they can reform the wizard, except in rare cases of self-defense. This is because Black Magic is almost always a slippery slope.
In Changes, Harry- after surviving a brutalTrauma Conga Line- decides that he will do anything to save his daughter, and that the ends justify the means. He ends up making a Deal with the Devil, and personally considers himself evil from that point on. Once his daughter is safe, he commits suicide, not wanting to live if it means being the Winter Knight. But that only makes things worse.
King Erius in Lynn Flewelling'sTamir trilogy starts by taking the throne from his insane mother, who was executing people left and right, in defiance of the divine edict that for no apparent reason essentially promises Bad Things if a man ever rules the country. Bad Things happen. You could debate whether or not he is really to blame for all that, but then he proceeds to institute sexist practices and start killing off his all female relatives.
In Memory, Sorrow, and Thorn, this trope in a nutshell is the backstory of the Big Bad, the Sitha prince Ineluki. Once a purely heroic figure, his ambition and willpower darkened when the Sithi's lands were invaded by savage humans. Dismayed by his people's despair in the face of their approaching doom, he delved into These Are Things Man Was Not Meant to Know and constructed a weapon so terrible that his father the king insisted he destroy it. Maddened by this rejection and by his torments, Ineluki murdered his father and took the crown, leading a final, futile resistance against the humans that ended in his death via Dangerous Forbidden Technique. It is deeply unfortunate for the world of Osten Ard that he did not stay dead.
Rachel from Animorphs spends three years trying not to fall into this as her Blood Knight tendencies slowly but surely turn her into the team's Token Evil Teammate. She has high points and low points, but generally she manages to keep it together until the events of The Return and the subsequent unmasking of the Animorphs by the Yeerks. Once she's abandoned the facade of ordinary life entirely what little restraint she had left quickly follows, and one of the last books in the series, The Sacrifice, consists of her mostly just kicking one dog after another. Realizing she's fallen into this, she agrees to a suicide mission at the end, knowing she could never fit into normal society again.
A Song of Ice and Fire: Theon was a jerk, but a pretty tame one by the standards of the series. However after taking over Winterfell and losing his hostages he allows Reek to kill innocent people in a cover up, including two children.
Live Action TV
Star Trek had numerous cases of this. For example, in Star Trek: Voyager, it's revealed that after giving the Hirogen holographic technology from Voyager, the Hirogen quickly got bored with the standard holograms and started creating more intelligent holograms to make their hunts better. These holograms eventually became self-aware and rebelled, before freeing others hologram in the area. Even Voyager's EMH joins their cause, agreeing that this is technically a form of slavery. Unfortunately, they quickly move onto anyone who uses humanoid holograms at all, regardless as to whether those holograms are actually sentient. The Doctor is horrified when their leader brutally murders a man to free the holographic equivalent of Clippy.
Teen Wolf: Lydia references this trope, warning Allison over the phone that Scott might fall off of it, starting with being thirty minutes late for dates and eventually ending up at domestic violence.
Xena: Warrior Princess had Najara, a character who either converted or killed criminals. Rather quickly, Najara is revealed to be insane, and can't tell the difference between obvious criminals and lesser offenders.
See also Calisto, who has a legitimate beef with Xena (Xena killed her family and wiped out her village), but every time she shows up she racks up more collateral damage and Disproportionate Retribution, becoming increasingly less sympathetic in the process.
A later episode reveals that Calisto, who has time traveled to the day her parents died, is the one who killed her parents and left her younger self to die. Note that none of this reduces her hate for Xena, showing that she doesn't even care at this point.
Season 2, "Bloodlust". The Winchester brothers met rogue vampire hunter Gordon Walker while looking for a nest of vampires. Gordon seems like a decent enough chap and a worthy ally, and Dean likes his "kill all the monsters and enjoy the hunt" philosophy. Dean and Sam end up fighting when Sam reveals that other hunters say Gordon is bad news. Before this can go any further, Gordon takes a swandive off the slope when the local vampires turn out to actually be peaceful, having sworn off killing humans, yet he still attempts to slaughter them. Then he tries to feed Sam to the head vampire to prove she's still a monster, and attacks Dean when they try to protect her. Bad move.
This is Castiel's entire character arc during Season 6. Desperate to defeat Raphael in the civil war in Heaven, Castiel begins performing many morally questionable acts, not the least of which is allying with Crowley, and rapidly skipping several shades of grey. This ultimately culminates in the season finale, where he jumps right into Villain Protagonist territory when he absorbs all the soul energy of Purgatory and declares himself the new God.
Whilst on a smaller scale, Castiel's ascension to God and later 'death' causes Dean to take a much harsher stance on supernatural beings throughout Season 7, most apparent in 'The Girl Next Door'.
Angel: During Season 3, Wesley translates a prophecy reading "The Father Will Kill The Son". Not quite sure how to handle the situation, he takes the baby away - for good - and even strikes Lorne unconscious when he finds out what's going on. To make that even worse, Wesley gets his throat cut and the baby taken away from him. And it was a false prophecy, anyway. Now Holtz has the child and takes him with him into a Hell Dimension, raising him to hate Angel.
In Season 6, Willow goes from killing Warren as revenge for Tara to trying to kill the uninvolved other guys from the Trio. Then to hurting Buffy, Anya (who initially sympathized with her), and Giles and finally to trying to end the world. All within a couple of episodes which together take place within less than one day.
The Initiative in season four was clearly using questionable methods in their study of demons, vampires, and other paranormal activities, but they were getting the job done and had effectively defanged Spike, one of history's most dangerous vampires. Then they decided that Buffy was a liability and tried to kill her. When it seemed like they were getting back on the slope, they took to torturing Oz (a good werewolf rather than an evil demon) and tried to kill the Slayer again.
Faith. When she first showed up, she had a lot of problems, not the least of which was that she enjoyed slaying a little too much, but she was definitely a good guy. Then she accidentally killed a man and the guilt (combined with all the speeches made to her about why she should be feeling guilty) made her snap and go NUTS. Later on this happens even more when she gets yelled at for her actions (such as when she tried to kill Angel or saves Buffy and an evil slayer).
Warren originally created a robot that would obey his every whim, but he eventually abandoned the android because he wanted a girlfriend that would be a partner in the relationship and he fell in love with a woman with her own ideas and personality. His creation of a Sex Bot and then abandoning it to "die" raises plenty of questions about his character, but he ultimately decides that he wants a woman that he can respect and interact with. In his later appearances in Season 6, he is a misogynistic bastard who tries to brainwash, and eventually kills, his ex-girlfriend because she would not submit to his desires.
Battlestar Galactica: The "Pegasus" arc has been accused of this by some critics, with Admiral Cain taking about twenty minutes to go from merely being a hardassed martinet to ordering the rape of a pregnant woman as a Jack Bauer Interrogation Technique and sentencing the crewmen who interfered to summary execution. In fact, the episode "Pegasus" had had to be radically trimmed to fit network time constraints; some of the footage that was lost (and reinstated on the DVD) implied the passing of more time than seems to go by in the episode as aired. The Razor movie, which came later, gave Cain a Psycho Lesbian backstory to explain her excesses.
The betrayal of Gina didn't explain her excesses; she shot her XO in cold blood for disobeying orders in battle before she learned her lover was a Cylon. She was just that hard, which also allowed her to quite impersonally order the torture and interrogation of said Cylon. If there's a Freudian Excuse in "Razor", it's that she failed as a child to protect her little sister because she was too scared.
Exactly, it's very unsubtly hinted at that Cain had been this way since the fall of the Colonies, which Razor confirmed, and for that matter implied she probably had a borderline personality even before the world ended. It just took a little time and provocation for the crew of Galactica to realize she was a Complete Monster.
There's also the episode where the woman put in charge of a tribunal takes about 24 hours to go completely nuts with power, and attempt to accuse the commanding officer who appointed her of the crime she's investigating.
Similarly, in the 'Black Market' episode, the leader of the organisation running it does a pretty good job of defending the need for a Black Market in the fleet. Then he talks about having child prostitutes, so Lee can shoot him without feeling guilty
Lee did acknowledge the argument about the need for a black market, though, given that he allows it to stay in business afterwards. He just wanted them to clearly understand where the Moral Event Horizon was.
Seems to happen about once a season in Doctor Who. A few notable examples;
In The Mind of Evil, a scientist invents a machine that removes criminal impulses from the human mind, and offers it to the government as a means of dealing with dangerous criminals without resorting to the death penalty. Turns out its inventor is actually the Master and the device brainwashes people to serve him.
In Genesis of the Daleks, Davros invents the Dalek (or "Mark III Travel Machine", as he initially calls it) ostensibly for the purpose of making life easier for mutated Kaleds. When his superiors start getting cold feet about the research, he has the entire Kaled race wiped out.
The audio drama Davros, released much later, showed that Davros was already lying in a heap at the bottom of the slope by this point. Not hard, when you're the leading scientist of a race of ersatz Nazis...
The original Daleks in "The Daleks" do this too. They're paranoid and threatening, but as the result of a nuclear war with another race that devastated their country and turned them into mutants incapable of surviving outside of travel machines in a specially-built environment with metal floors. While they trick the humans into it, all they really want is anti-radiation drugs which would allow them to leave their suits. It then turns out that their bodies have adapted to need radiation and the withdrawal sends them mad before killing them, so they decide to shoot out a load of radioactive waste onto the planet again when they realise they couldn't survive without radiation.
In "Rise of the Cybermen", when the British government refuses to fund John Lumic's Cyberman research, he kills the leadership and begins forcibly cyber-converting the British population.
In "The Unquiet Dead", gaseous beings called the Gelth need to animate human corpses to house themselves and hence survive — creepy, if not evil. They ask to come to Victorian Cardiff, and the Doctor, dismissing the Squick of his companions, agrees. After the Gelth come through, however, it turns out they lied about their numbers and intentions. They want to take over all of Earth's living bodies — but even before we learn this, we can tell that they're malevolent, because shortly after getting the Doctor's go-ahead, they switch from pale blue to bright red and Satanic. Apparently, they were "demonic" all along, see?
"Partners in Crime" begins with an alien conspiracy that... helps people lose weight effortlessly by giving them pills that cause one pound of fat to turn into an adorable little creature called an Adipose every night. While this comes off as slightly sinister, it's hard to see how they could ever be an enemy- until, of course, the "breeders" of the Adipose decide that their current method is too slow and try to make Adipose out of the entire body of their victims, killing them in the process. Jumping? More like a great, flying leap.
The Doctor himself jumps from the top of the slope to the bottom in the last 10 or so minutes of "The Waters of Mars". After having spent the whole episode with a group of people destined to die, the Doctor snaps and decides to save them after most of them have already been killed. Just moments after saving the remaining people (in the most epic way possible), The Doctor decides that he can mess with the timeline in any way he sees fit, completely ignoring his species' laws. At this point, the Doctor is almost antagonistic. Thankfully the Doctor is only in this state for one scene, and is brought down a couple of pegs before he can really do anything.
It's implied in "The Runaway Bride" and more-or-less stated in "Journey's End" that the reason Doctor travels around with a companion is so that he has someone to remind him not to do this, since he can so much power and gets into such intense and painful situations it would be hard for him not to slip, and hard for anyone to stop him once he starts sliding.
Holly in Slings and Arrows wants to streamline the Festival's business end and replace most of its William Shakespeare with musicals. This is only marks her as a villain in the context of a show where Shakespeare is Serious Business, until she starts abusing her boyfriend and deliberately aggravating the heart problem of a board member who disagrees with her.
The Ori could stray into this. At first it seems that, while their practices are primitive, their ultimate goals are noble enough, helping others to achieve ascension. Then it's revealed that this is all a lie, and the Ori are manipulating people's belief to gain more power.
The rogue NID. At first they're stealing alien technology with the purpose of using it to defend earth, making them into Knight Templars. Then it turns out they're just in it for the money.
Similar to the Magnum Force example, season two of Murder One featured a storyline about Clifford Banks, a serial killer who tracked down and executed criminals who escaped justice, or had an unsuitably short prison sentence. He started out on this path through the murder of his retarded brother, he never kills innocent people, and throughout the arc a few people comment that "sometimes the streets need sweeping." Any moral ambiguity is then done away with by the lawyers finding out that Clifford actually killed his brother himself over his frustration about giving up his whole life to care for him, causing a mental breakdown that directed his guilt outwards onto other criminals.
In the pilot episode of The Shield, Vic Mackey partakes in numerous criminal acts including the use of excessive force during arrests, working with a drug dealer and beating a suspect with a phone book in order to make him talk. Then, at the end of the episode, he shoots another police officer in the face to prevent him from gathering evidence against Vic's team.
An episode of The Commish features a Vigilante Man who initially only humiliates bad guys who deserve it. But when an accused rapist/murderer is found not guilty (for good reason), the vigilante (who believes he got Off on a Technicality) clubs him to death, setting the team on his case.
The season 8 finale of Smallville took an incredible amount of heat for various reasons, and one of them was this trope.
Tess Mercer established an alliance with the Kandorians to save Earth from mankind's destructive ways but in the Bad Future, she collaborated fully with Zod's despotic rule over a dying human populace.
General Zod's interest in restoring his and his fellow Kandorians' powers (against Clark's objections) made sense in light of the repeated violent threats they faced from Amanda Waller and other humans Properly Paranoid about aliens among them. After Zod and the Kandorians get their powers restored, he destroys Checkmate, kills Faora (and their unborn child) for going against him, and plans to take over Earth with the Kandorians loyal to him and make it into a New Krypton.
Several characters in the 1998 Merlin series, but most notably Uther and Mab.
Likewise, in the later Merlin BBC series, Morgana was understandably angry and bitter, but nevertheless sympathetic. However, between seasons two and three, she transformed into a smirking villain.
Although it's partially justified by the fact that she spent a year with her sister in between seasons two and three. Plus, Merlin poisoning her to save Camelot can't have been a good thing for her judgment.
In one episode of Monk, when Captain's Stottlemeyer's wife is gravely injured in the fallout of a union assassination, Stottlemeyer proceeds to teeter dangerously close to the edge in his hunt for the sniper. Near the end, he very nearly launches a raid on the suspected union until Monk manages to crack the case.
Fringe: Walternate originally just wanted to save his universe, even if it meant destroying a parallel universe and its inhabitants. Then he attempted to kill his son and the mother of his grandchild.
An episode of MacGyver involved a business owner attempting to have the Challengers Club shut down because one of its members stole a truck from his printing business. What could have been a two sided conflict between a racist business owner - albeit one who had a legitimate axe to grind - and a teenager conditioned by poverty and racism to view white people as the enemy shifts step by step into a case of the boy being a clear cut victim of The Man. First it turns out the business owner framed the kid for stealing the truck as a pretext to have the Challengers Club shut down. Then he escalates to murdering the club owner. Then it turns out he prints white supremacist propaganda and thinks "niggers should be drowned at birth".
24 does so respectively with Tony Almeida and Jack Bauer in its final two seasons. In Tony's case, After his wife and unborn son were killed, he sets out to kill the mastermind in any way possible. But tactics he employs in trying to do so include things like murdering the director of the FBI, nearly exposing several innocent people to a lethal pathogen, and attempting to sacrifice Jack in order to get close to his target. As for Jack, after years of being tortured, screwed over, and having friends and loved ones taken from him, he finally loses it when Renee Walker is killed as part of the Russian's cover up and the President betrays him by refusing to reveal the truth since exposing them would also expose their involvement in the murder of a foreign president, which in turn would ruin the chances of a peace treaty she's trying to have signed. Jack claims that he'll take justice into his own hands and expose the truth, but his doing so involves murdering The Mole solely because had been working with the Russians beforehand long before there was any movement made to kill Renee, slaughtering several members of the Russian government, opening fire on a crowd of innocent people (which although it was mainly to disable random pedestrian cars, it's still made perfectly clear that Jack could have easily killed someone with even the slightest slip-up and really didn't give a damn at all) and attempting to assassinate both Yuri Survarov and Charles Logan even though killing the both of them would be guaranteed to start a war between the USA and Russia that would likely lead to the deaths of millions. The series does its best to make sure that what he's doing isn't in any sort of heroic light.
Charmed has a twist on this in Season 2's "Morality Bites"; after the sisters use their powers to pull a vengeful yet relatively harmless prank on a man who continually lets his dog defecate on their driveway, Phoebe has a premonition of being burned at the stake for killing someone with her powers a decade in the future. One adventure later, they are sent back to that point in time to prevent falling prey to this trope, worded well by Phoebe:
Phoebe: Once you break the small rules, it's just a matter of time before the big ones are next.
Hannibal: Will goes from doing some morally ambiguous, but still understandable things, (such as trying to have Hannibal killed) to something unambiguously evil in the episode ''Naka-Choko".
Of course it turns out to be a con to convince Hannibal that he'd gone slope-jumping. And it worked, for a while.
Vampire: The Masquerade has an actual mechanic for this: acting like an inhuman, unprincipled bastard will make you more of an inhuman, unprincipled bastard.
This applies to all World of Darkness games and is a large part of the new system.
The old system was an aversion; the more humanity you lost, the harder it was to lose the next point, the more extreme your behavior would have to be. Only if you're determined to destroy your humanity (or your Gamemaster paves your path with Sadistic Choices,) could you slip past a certain point, but it wouldn't happen by accident.
It wasn't supposed to be an aversion because as the character's Humanity dropped, the character's sense of morality did as well. Yes, by the time you're Humanity is down to 3, say, it took doing something truly depraved to drop it any further, but by the time you're down that far, you consider murder to be roughly the equivalent of breaking the speed limit, so there isn't much keeping you from going even farther. However, since in reality a player character's morality is determined by the player, in practice this rarely came into play.
The entire theme of the Alpha Legion in 'Legion'. They are a secretive legion who achieve victory through the best means necessary, even if it means the deaths of hundreds of Guardsmen, but are still loyal to the Emperor. However, at the end of the book, they join the forces of Chaos, believing it's what the Emperor would want. Though it is possible they only wanted it to look like they had jumped off the slope so that they could manipulate Horus into losing the war.
Tau as well, when one considers that its for the greater good for sterilization policies, and special "helmets" for their bug allies.
The Inquisition contains two major factions: Puritans and Radicals. Puritans are the standard "burn the planet to ashes if there's a hint of Chaos on it" guys, while Radicals are the ones willing to use chaos against itself (i.e., get a guy possessed by a demon so he can use stupidly powerful magic or wield possessed weapons). Strangely, Radicals tend to be older than Puritans, it's implied seeing a lifetime of fighting against Chaos have so little effect they start using The Dark Side. Pretty much all of them end up falling to Chaos anyway.
Then there's the demons, dead and raksha themselves, who tend to be less evil and more along the lines of a Blue and Orange Morality. Demon summoning is a fairly common practice for everything from construction to medicine to entertainment, some places in Creation have regular interactions with the denizens of the Underworld, and The Fair Folk make great trade partners because of their ability to buy and sell immaterial concepts (like dreams and emotions).
Played straight with the akuma, who allow themselves to be completely remade as agents of the Yozis in exchange for power. However, given the circumstances that lead many to become akuma, they tend to be a Woobie, Destroyer of Worlds.
Injustice: Gods Among Us has a lot of this: Superman doesn't just jump, he literally flies in the highest speed he can possible throughout the slope after being tricked into killing Lois and then killing Joker in retaliation, establishing a totalitarian dictatorship where in exchange of his "protection", everyone that even dares to protest his method get swift death, as what happened to that universe's Green Arrow and later Shazam, and according to the backstory, Hawkman, causing Hawkgirl to retaliate in vengeance, only to be Brainwashed into servitude. Via backstory, we see the only surviving Teen Titans being Cyborg and Raven, both end up disillusioned and Raven ends up giving in to Trigon's influence, gaining lust on torture and becoming Trigon's worshipper instead of trying to prevent his coming. And while Damian Wayne did accidentally kill Dick Grayson, he didn't look back in regret and goes far worse than before because Superman, being his 'new father figure', got to convince him to stay in the extremist way. One man jumping off, the others follow suit in varying level.
In BioShock, harvesting more than two of the Little Sisters gives you the bad ending; it is simply implied that you jumped off the slope and became ADAM- and power-hungry the moment you first harvested.
This is actually justified — killing the Little Sisters gives you more ADAM, and why should you be immune to the Psycho Serum that's turned the rest of the city into twisted freaks?
By that logic, wouldn't the Psycho Serum force you to harvest the rest of the sisters Controllable Helplessness-style, if harvesting even the first two sisters is enough to give you the evil ending?
And yet, if he gets the same amount of ADAM from Tennenbaum, or just from rescuing four Little Sisters, he's fine. Methinks the ADAM didn't have much to do with it. Rather, it's that killing children just to get a slight power boost could easily be seen as crossing the Moral Event Horizon.
Let's not forget Andrew Ryan. The whole point of Rapture was to create a utopia where individuality and free enterprise were unrestrained by the government. Once Fontaine began to rise in power though, paranoia and a fear of losing his city turned him into an ironfisted, totalitarian dictator, the exact opposite of what he set out to become.
And in BioShock 2, if the player jumps off the slope so does Eleanor.
In Neverwinter Nights, Aribeth leaps quite quickly down the slippery slope (partially excused as Morag is messing with her brain and her intentions)
CJ and Niko Bellic from Grand Theft Auto: San Andreas and Grand Theft Auto IV, respectively. Let's assume that they're good-hearted people at the start (if the cut-scenes are any indication), and let's assume the player doesn't do any killing not encouraged by the storyline (which is a stretch, but go with it). Now watch how their lives unfold. CJ in particular goes from "I guess I'll kill this guy since he's been screwing with my gang" to "guess I'll just kill all these guys for no apparent reason" so quickly it might make you wonder if you're still playing as the same guy.
Well, Niko may seem pretty nice at the beginning of the game, but the plot eventually reveals that he is a war criminal out to kill other war criminals. So there's a good argument that he starts the game as a major bad guy, and indeed committed even more horrible acts before the game started than you can ever do in it.
Mega Man X 8 has Lumine, a New Generation Reploid, and director of the Orbital Elevator project. He's the Big Bad, not Sigma this time.
It doesn't help that the whole of Lumine's tale plays on the game's subtitle, Paradise Lost. Lumine is the analogue to Satan, rising against his creators and their vassals. He even seems to have enough truth in his words to shake up X into being completely unable to attack.
Malygos from World of Warcraft goes from a dragon who wants to rein in mortal spellcasters because he disapproves of their methods to a dangerously extreme tyrant who seems genuinely unaware that his plan to redirect and control magic has an excellent chance of destroying Azeroth.
Kael'thas Sunstrider's goal was originally to improve his suffering people. As time went on he began to make more and more questionable alliances, first with the naga, then the partially demonic Illidan, and finally knowingly aided the purpose of Kil'Jaeden and the Burning Legion (albeit only helping them to fight Arthas, which was an admirable goal no matter who wanted it). And then he tried to summon Kil'Jaeden so the Burning Legion can destroy Azeroth, killing his own people when they tried to stop him. What.
Illidan was always a self-serving Jerk Ass, but he had a more gentle side to him. After nearly being killed by Arthas, though, that gentle side was replaced with paranoia, insanity and a desire to crush anyone he deems as a threat, which happens to be everyone not on his side.
This happens every time one of his plans fails though. Logically all that needed to happen was for Malfurion to beat on him for a few minutes and Tyrande to be in danger, and he'd do the same thing he's done every other time. Switch sides to save her.
And, while we're on Warcraft games, as you play the human campaign of Warcraft III, Arthas starts out as a dedicated disciple of Uther Lightbringer (even though Arthas is a prince, Uther's military rank is higher than Arthas's, and they both respect that) but gradually starts betraying more and more people, and becomes less and less concerned with the whole reason he's fighting the Undead in the first place. Eventually, he totally betrays Azeroth, dresses in Undead armor, and kills his own father. In fact, in World of Warcraft, the very throne room in which he killed his father is now directly above Undercity, the Capital City of the Undead. In his case his abrupt descent was exacerbated by him taking up a cursed sword that stole the soul of anyone it touched.
After Deathwing, the Big Bad in the Cataclysm expansion, is ultimately destroyed, the Horde and Alliance turn on each other big time in the following expansion, Mists of Pandaria. In particular, the Horde Warchief, Garrosh Hellscream, is showing signs of going off the deep end, and is currently forecast as the end boss of Pandaria.
Arcturus Mengsk of Starcraft started out as a dashing rebel leader who saved you and Jim Raynor from the Confederacy for killing Zerg. The first time he used a psi emitter to summon the Zerg it was a military target and the rebels helped the majority of civilians flee. Then he dumped several on Tarsonis, a planet with a population of two billion, before attacking the Protoss who came to stop the Zerg, using the orbital defenses to stop anybody from fleeing, and abandoning his second-in-command to the Swarm.
In Mitadake High it is common for someone to RP themselves going insane as a result of the madness going on around them. Unfortunately, not everyone is any good at it.
In Fire Emblem: Path of Radiance, the country of Crimea is good with bits of gray, Daein is dark gray but with evil leaders, and conservative Begnion was in the middle with its corrupt Senate but well-intentioned leaders. When the sequel rolled around and Begnion became be the main antagonist, it became more ruthless.
Final Fantasy VII - Sephiroth is initially the best SOLDIER in the world, but after finding out a certain fact about himself, he becomes a murderous psychopath, slaughtering the population of a village and burning it to the ground, and then sets out to destroy the world.
In Final Fantasy XIafter the woman that he loved was killed and he was left for dead by Ulrich during the Multinational Expedition to the Northlands, Raogrimm kills Ulrich. Then he hunts down and murders the rest of the people in the Multinational Expedition because they knew that Ulrich had done something and didn't say anything about it. Then he gets a giant "Slip 'N Slide" and whisks down the slope gleefully as he declares war on the human nations and nearly destroys the world. Mind you, some of it may have been the Dark Divinity Odin fanning the flames of his rage, but still... Although, Ulrich's actions during the Multinational Expedition could be considered the ultimate slippery slope, since they were the cause of pretty much all of the major, world-threatening troubles that Vana'Diel has faced in the following 30 years were stemmed from his (accidental) murder of Cornelia.
So many in Dragon Age II, a game where no one is really evil and no one is truly good. By the end of the game, both of the leaders of the two warring factions give into their inner demons with Meredith, the Knight-Commander of the Templars calling for the execution of all mages in the city of Kirkwall for the actions of just one rogue mage who also jumped off the slippery slope and First Enchanter Orsino, leader of the mages, using Blood Magic in an act of despair.
Both slopes were greased with phlebotinum in this case; Meredith was being corrupted by the lyrium idol in addition to her own paranoia, and the rogue mage was possessed by a demon of Vengeance.
Actually, in the case of : Orsino he was not possessed by a demon but instead used Blood Magic to turn himself into an abomination of human flesh. The rogue mage was actually Anders, who corrupted a spirit of justice and turned it into of spirit of vengeance, and depending on your interaction with him was either possessed, or willingly blew up the chantry and single-handedly plunged the world into war. Because he willingly allowed the spirit to possess him, its debatable when, and how many times, Anders went off the slope as he is a well-intentioned extremist throughout the game, but before, in Dragon Age: Awakening, there was little, if anything, to hint that he would even become remotely extreme.
The Protagonist from the Saints Row series gleefully leaps headfirst off of the slope, and then proceeds to nuke it. In the first game, you start off as a (mostly) silent henchman who more or less indifferently does what Julius, Gat, Lin, Troy, and others tell you without hesitation, and you seem to be a pretty sane individual. While you are killing, you're killing the other gangs for peace, and the cops you kill are corrupt anyway (of course, not counting civilian casualties in your gameplay rampages). But in Saints Row 2, after being betrayed by Julius and being blown up and disfigured to the point of needing severe plastic surgery (which is really just an excuse to make a new character), it's implied that you went insane and very much stated that you're paranoid, corrupt with power, take deep pleasure in murder, is only after the city, and nothing short of evil- the only people outclassing you are the gangs you fight and their leaders, but not by much. As the game goes on, it becomes clearer and clearer that you're not very interested in wiping out the city for peace anymore as your actions become more and more violent and Crazy Awesome, especially after two of your homies get murdered. The only person who ever stood a chance of stopping you, your old boss Julius, turns out to have done it because he savililyrealized that you were a dangerous person; you kill him while happily stating you have full intentions of taking over the city in any means necessary.
Johnny Gat qualifies as well.
Saints Row: The Third plays this with most if not all of the main characters, and they each suffer for it. Boss, Gat, Shaundi, Loren, Killbane, Kiki, Temple and Kia are just some of the names who are guilty of this, and all either die or with the exception of Boss can be killed. Boss arguably gets it even worse if s/he chooses to jump off the slippery slope: s/he reverts back to being worse than ever, and a thoroughly despicable person.
The Illusive Man from the Mass Effect trilogy takes a flying leap off the slope in Mass Effect 3. Whereas before he was a Well-Intentioned Extremist who tended toward a lot of Shoot the Dog moments in his zeal to protect humanity, in the third game he flies straight off the rails and starts using Reaper technology to assemble a massive army of Brainwashed and CrazyMooks, ordering the murders of civilians, and actively working to undermine the Alliance and the Council in their efforts to defend the galaxy against the Reapers. Eventually, it's revealed that he has completely hurdled the Moral Event Horizon with Sanctuary, a supposed safe haven for refugees from the Reaper attacks, which turns out to be a laboratory where the refugees are forcibly converted into Husks as part of his research into finding a way to control the Reapers.
Explained by the fact that he was indoctrinated by the Reapers for the entire game.
In The Order of the Stick paladin Miko Miyazaki starts out as a narrow-minded, Holier Than ThouKnight Templar who the titular Order despise and even her own comrades tend to look for excuses to send her off on missions to distant lands that keep her out of town for long periods. Then she overhears Lord Shojo talking to Roy and Belkar about their plans to do the dirty work behind the paladins' backs, ignores his perfectly good arguments about why he had to do it, declares him guilty of treason and executes him on the spot. She's IMMEDIATELY stripped of her powers by the gods for murdering an unarmed octogenarian and goes into a psychotic breakdown when she refuses to accept that she could have been wrong.
Vaarsuvius took a jump, too. See comic #639. Though debates on whether this counts as Pay Evil unto Evil, and whether that stops it being this trope, rage on the forums endlessly.
As said by the fiends, the best way to get a good person to do horrible things is to convince them that they aren't responsible for their own actions.
Wanda from Erfworld. Ever since she attuned to the Arkenpliers, she has become more and more sadistic and cruel, to the point that, when the team's Foolamancer is injured and unconscious, she says they should kill and zombify him instead of healing him, simply to save on resources.
It ends up being subverted in that Jack knew something that Parson really needed to know, but Jack was contractually obligated not to tell anyone for the rest of his life.
The inspector in Chisuji. First he decided to take justice in his own hands against the criminal who killed his wife and put his daughter in a coma; then he saw the killer's girlfriend holding said daughter's plush toy, and... snapped.
The Batman: The Animated Series episode "Lock-Up" introduced Lyle Bolton, ruthless head of security at Arkham Asylum, who eventually goes crazy and becomes the supervillain Lock-Up. He starts off making some good points about his regime bringing Arkham's role as a Cardboard Prison to a halt. Fortunately - so to speak - he also turns out to be a sadistic monster who steps way past his boundaries, abuses his inmates, and eventually starts locking up politicians and media members, blaming them for allowing crime to run rampant in the first place, allowing Batman to take him down without any worries.
When new-vigilante-in-town The Judge shows up later on, attacking the villains and not caring whether or not he kills them, this is never even brought up. It is taken for granted that his actions are wrong, which (given the long, horrible careers of Batman's rogues gallery) seems like it would be open to debate here. The big jump probably comes moments before Batman intervenes, when he is about to kill a small-time corrupt politician who had helped him, but still. The Judge showed how extreme he can really get when he tried to kill Two-Face in his own escape room. As it turns out, The Judge is Two-Face, as he is a third persona made by Harvey Dent to fight crime.
Commissioner Gordon in "Over The Edge". After his daughter is killed in a fight with the Scarecrow, he blames Batman and launches a manhunt for him, going as far as to make a deal with Bane. Fortunately, it was All Just a Dream.
Also, in Justice League, Cadmus. Their stated goals: Provide America (and her allies, probably) a defense against the super powered types, especially the Justice League. What with Superman nearly taking over the world when being brainwashed by Darkseid, the Justice Lords in a parallel universe taking everything over, and the Justice League having an Orbital Superweapon pointing down, this seems entirely okay. Up until the cloning, torture, firing nuclear weapons, being allied with Luthor, creating Doomsday...
What made Cadmus utterly irredeemable was finding out that they were responsible for putting Ace (the youngest member of the Royal Flush Gang) through hell, robbing her of having a halfway normal life and, eventually killing her by overloading her brain to evolve her psychic powers, and triggering a fatal aneurysm in the process. However, she died naturally after Batman went to be with her in her final moments.
There is also the fact they tried to blow up the Watchtower before the League had ever done any harm, and that Gen. Eiling was willing to a nuke an island to "kill three birds with one stone," i.e. kill both Superman and Doomsday and stop the drug smuggling that came through it. Granted, only Eiling was behind this, and Amanda Waller is furious as soon as she finds out about the nuclear air strike.
And the Justice Lords from a parallel Earth. Superman abandoning Thou Shalt Not Kill to stop Luthor from starting a nuclear war: justifiable. The entire team doing away with the concept of Joker Immunity altogether and resorting to killing and lobotomizing on a semi-frequent basis: arguable. Setting up a totalitarian state in which elections do not happen until the Justice Lords say they do and people can be arrested for complaining too loudly: seems unnecessary.
And for that matter, Doctor Destiny's origin story in "Just a Dream". At first, he seems to be a fairly decent guy whose big mistake was simply getting hired as a guard by Lex Luthor, and the story starts raising questions about What Measure Is a Mook? and the hypocrisy of the henchmen going to prison and having their lives ruined while the villains themselves keep getting away scott-free. But once he gains superpowers himself, his Roaring Rampage of Revenge throws him right off the slippery slope, and into territory that even Lex never touched, with lemming-like gusto.
Jet's goal it is to protect children like himself orphaned by the war mongering Fire Nation and to fight back. However, it's made pretty clear that Jet has jumped off this slope when he attempts to drown an entire town uninvolved with the war effort, murder innocent elderly people, and put his own life at risk for the purpose of revenge. He notably later attempts to jump back on the slope, but it doesn't turn out too well.
It's implied he had already fallen that far long before they met him- the plan was already in place and he had apparently been attacking travelers indiscriminate to their threat-level for a while now. Not to mention his "enforcers" thought nothing of Jet ordering them to kill Sokka. It's not entirely clear whether he truly regretted his actions for being morally wrong.
This is what leads to Zuko's eventual Heel-Face Turn. He'd been hesitating for a while, thinking that his family really were good people, despite all the massive evidence otherwise: it's when his father and sister plan to burn an entire country to the ground that he realizes they've jumped off.
Tarrlok starts out as a Jerkass and manipulative self-serving politician. Then he goes completely off the deep end imposing a curfew on all non-benders and arresting anyone who complains or even has connections with Equalists. He arrests Korra's friends to blackmail her to join him and when she refuses, attacks her and reveals himself to be a bloodbender. By the end of the episode, he's got her locked in the back of a Satomobile to take her somewhere she'll never be found.
Amon and the Equalists start off with a relatively valid complaint: Benders really do have all of the power in Republic City. Up until Episode 10 they had only committed one really extreme crime. Then, they jump full on off the slope by launching a full-scale invasion of Republic City, complete with bombings and gas attacks!. Arguably, their point is also undermined by the fact that Amon is actually a stupid-powerful Bender himself and uses his own bloodbending ability to destroy others' ability to bend, but whether that's this trope or simple Hypocrisy is open for debate.
Each member of Team Impossible; went from just being a Punch Clock Hero who wanted Kim to stop saving the world so they could get paid for it, and basically straight to trying to permenantly end her heroics.