Slowly Slipping Into Evil

A recurring scenario where a character, after making a small evil act, finds himself becoming more and more evil, very often far more than he originally planned or intended.

Sometimes you can see what you want just beyond your reach—you don't have to jump off the slope, and it's not all that steep anyway, just a few steps on The Dark Side and you'll have it. Yet what was just a short walk in one direction is not so easily travelled in reverse. Even if you turn back immediately afterwards, you find yourself slipping down the slope faster than you can climb your way up! Eventually you may tire of fighting it and just let yourself continue to slide unhindered, or even worse: choosing to embrace the inevitable and turn your steps in the direction of your momentum.

This can be done for different purposes:

A warning that Evil Feels Good, and that doing some evil, no matter how good an idea it seems, is always a bad idea. When Evil Feels Good thanks to Phlebotinum, that's The Dark Side. The combination of these two tropes is very likely to result in The Dark Side Will Make You Forget.
A warning that once you commit an evil act, no matter how small, you are forced to commit more and more evil.

For obvious reasons, almost mandatory in Start of Darkness stories. See Moral Event Horizon for when someone commits an act so heinous and unforgivable that he is now unquestionably evil. See also Had to Come to Prison to Be a Crook, which is when one of the reasons for falling is spending time in jail, and This Is Your Brain on Evil, for when performing evil act is akin to drug intoxication, and thus very likely to provoke this trope. Do not confuse with Jumping Off the Slippery Slope, which is when someone on the same of the side of the heroes but on a morally gray path commits an evil act to prove the rightness of the heroes' path.

Supertrope of He Who Fights Monsters, wherein fighting against evil is the thing that causes the person to slip into evil.

Examples of Slowly Slipping Into Evil include:

Anime and Manga

Lelouch and Suzaku in Code Geass become more and more radical during the show's progress, although their goals were noble.
Naruto:
- Sasuke didn't just slide down the slippery slope, he grabbed a sled on the way to the edge so he could get down faster. First, he betrayed the village by joining Orochimaru just so he could get strong enough to kill Itachi. He at least had some qualms about killing at this point. Later on, after killing Itachi, he learns that Itachi massacred their clan under orders from four of the higher ups (one of which was against it but overruled) in order to stop a civil war that could develop into a world war. Naturally, Sasuke decides that the entire village has to die because of this. By the time he reaches Konoha, he is not above cold-blooded murder of even his allies just to accomplish his goals.
- Ridiculously enough, he gets worse! Regardless of his statements of genocide, he at the very least tried to protect and save the friends that helped him on his revenge quest. Now, he's (completely out of nowhere) gotten to the point that anyone who so much as inconveniences him or his goal even slightly must die. Karin, who not 50 chapters ago he risked his life for, got caught and held hostage by Danzo, so what does he do? He spears them both instantly without a second thought. What's this, Karin didn't die? He tries to finish the job. What, Sakura, Naruto and Kakashi are here? Might as well try to kill them too.
Light from Death Note goes from dark Anti-Hero to full Villain Protagonist during the course of the series (going from killing criminal to killing lazy people). It's implied that the eponymous Artifact of Doom is the cause of this.
In Saint Beast, Zeus' slip starts with overthrowing the old gods which he feels a measure of regret about, but thanks (in part) to Lucifer's support, by the time Judas suggests purging the evil angels in heaven Zeus has basically lost it and keeps getting worse.

Comics

Winnowilll in Elf Quest is initially at least reluctant to kill to further her ends. Ironically, her descent into cold-blooded murderess seems to be at least partially due to a botched mind-healing attempt by Leetah.

Film

Michael Corleone in The Godfather.
Anakin started to feel doubt and hatred in Star Wars Attack of the Clones, killing an entire population resposible for killing his mother (regardless of individual involvement). If not that then Sidious manipulating him into killing Dooku probably marks the pivot, and the children's slaughter the end of the slope.
Peter Parker in Spider-Man 3 slides very slowly. First, he combs his hair differently, then he refused to pay his rent to his landlord until his door is fixed, and after that he finally perform evil acts, such as manipulating people and punching them (accidentally).
Stephen in The Ides of March goes from idealism to cynicism and worse.
Magneto in X-Men: First Class; a really good and interesting example in that he starts the movie already as an Anti-Hero and spends most of the movie establishing his attitude that would cause him to slip into evil before he actually does so. And while he's undeniably a Jerkass in opposition to the heroes by the end of the movie, he still isn't evil and never really crosses the Moral Event Horizon.

Literature

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 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. He is at the start of the story the Big Bad and an Omnicidal Maniac.
Conall Haldane in the Deryni novels The King's Justice and The Quest for Saint Camber goes from merely being a Royal Brat and Sore Loser to committing murder and treason. He says as much at his trial: "I didn't start out to betray you, Kelson," he sobbed, "but things--happened. It wasn't fair!"

Live-Action TV

In Merlin, Morgana has a long-term one of these over the course of the second and third series. Of course, this is usually conveniently forgotten about when she is not directly the antagonist of each plot of the week.
Lex Luthor has this happen to him over the course of Smallville going from close friend of Clark's and genuinely good guy to evil villain over the course of the series.
- Lampshaded when he tells Clark that 'Evil is a journey, not a light switch'
Gul Dukat on Star Trek: Deep Space Nine was always a nasty authoritarian, but in early seasons would help the good guys on a pragmatic basis. After the Dominion War started, he aligned Cardassia with the Founders. When that didn't work, he became a full-on Omnicidal Maniac.
The whole point of Breaking Bad. Walter White starts as a chemistry teacher who gets cancer, so he begins to make meth as a way to leave money for his family. Then, his actions become not only much less justified, but also he goes on to kill and begins some big schemes.
Jimmy Darmody from Boardwalk Empire has a conversation with his wife in the second season about how every time he draws a line in the sand, he winds up having to cross it to be a successful gangster. This eventually results in him doing all sorts of things he never thought he'd do, and that other people are horrified by, including ordering the assassination of his father figure Nucky.

Video Games

A major theme in BioShock (series), where the two Big Bad during the backstory change from rude ruler to the people they hate most (in the first, an objectivist became first a dictator, then a totalitarian dictator, in the second, an Altruist became more and more selfish and end up sacrificing everything to save herself). The Aesop here is the danger of fanaticism.
In Mafia: The City of Lost Haven, Tommy starts off trashing a rival family's cars and finishes the game as a stone-cold killer.
Kefka from Final Fantasy VI starts as a Harmless Villain, but finishes as an Omnicidal Maniac Godlike Big Bad.
Arthas in Warcraft III is initially a Well-Intentioned Extremist when he decides to kill the citizens of Stratholme who ate infected grain which turns humans into zombies and would soon morph into the undead. After unwittingly selling his soul to the Lich King in order to make use of the sword called Frostmourne, he kills his father, the king of Lordaeron, destroys the elven capital of Quel'thalas, and aids in the opening of a demonic portal for the Burning Legion.
In World of Warcraft, the Horde under the war-mongering racist Garrosh Hellscream is slowly becoming The Horde again (though it's not quite that bad yet), and this time they don't even have the excuse of Fel corruption. The Forsaken under Sylvanas have become more explicitly villainous after Sylvanas experienced The Nothing After Death which drove her to basically turn her Forsaken into the Scourge 2.0 in a bid to avoid death forever. Even Garrosh thinks she's going too far.

Web Comics

In Book 9, Part V of Schlock Mercenary, General Xinchub is revealed to have gone through one of these:

Xinchub: I sold my soul a long time ago, believing that I was helping Humanity, and all our Terran cousins. I've done all kinds of atrocious things, and somewhere along the line I started to enjoy them.
Ceeta: Happy memories?
Xinchub: Yeah, good times, good times.

Web Original

Doctor Horribles Sing Along Blog - the eponymous character starts off as an ineffectual Anti-Villain, maybe with good intentions, but through constant humiliation, losing his love interest to the superhero Captain Hammer, and his own urge to join the Evil League of Evil, he takes a long ride down the evil slope, culminating in the attempted murder of Captain Hammer.

Western Animation

Seth MacFarlane likes this trope, although he plays it for laughs. He's Flanderized two of his star characters, Peter Griffin of Family Guy and Roger Smith of American Dad, into Evilly Affable Jerkasses. Roger's Disproportionate Retribution to Steve for taking his cookie was arguably the start of his crueler aspects taking over his role, while Peter's probably began with the lighter abuse of Meg in early post cancellation episodes.
The Legend of Korra: Amon first targeted criminal benders. Next time he attacks, his targets are cheating professional wrestlers. Note that the latter operation included heavy material damage and endangered lives mostly due to Korra's counterattack. When no one fights back, equalists operations are very clean.