Villain Corner

When a certain (allegedly) non-villainous character starts getting explicitly dark character development, they are being painted into the Villain Corner.

This new development can be either subtle or blatant, gradual or abrupt. The character can be an old friend of the protagonist, a friendly acquaintance, a former Mentor, or any other type of automatically respected person; the character can even be the actual protagonist. It can involve the revelations of dark deeds from the character's past, or manifest in new actions or attitudes which go against the established grain. In any of these cases, the audience is not supposed to suspect anything evil about this person; indeed, there may not be anything evil about them, at least initially.

The key to this trope is ambiguity. Once a previously good character begins to undergo this treatment, the narrative typically will become increasingly mum about their actual motivations and alignment. This can serve as an unintentional (or intentional) Lampshade Hanging, as it is a sure sign that we're not dealing with the sympathetic character we started out with.

This tactic can be handled well, and can add nuance and intrigue to an established character, shaking up potential story arcs and fan expectations. But when executed poorly, a clever audience will have them picked out from the beginning like an oak tree in a beanfield, even if the other characters don't see it.

Compare Knight Templar and Well-Intentioned Extremist, which is where this trope often ends up. See also Anti-Villain.

Anime and Manga

 * in the first season of Higurashi no Naku Koro Ni approaches the corner very subtly, and they realize it very abruptly.
 * Quite a lot of Code Geass is about Lelouch trying not to paint himself into the Villain Corner... and failing miserably.
 * Sasuke. Originally he was just a slightly dark blue oni woobie, but the actual villains gave him a gentle push toward the corner, and, well...

Comic Books

 * While it's no secret that Cassidy from Preacher (Comic Book) was never a saint, beginning with "Dixie Fried" he started to act more bastardly than was common up to the point, and then came "All Hell's A-Coming" where we find out just how bad he really is, and for how long.
 * The Ultimate version of, who had three interconnected miniseries dedicated to going off the deep end.

Film

 * Professor Xavier in X-Men III: The Last Stand
 * Except Xavier was right. Jean Grey was the correct instance of this trope.
 * This is more accurate in the comics, particularly the story arcs "Deadly Genesis" and "Dangerous," both of which seem to paint Charles as being both a Well-Intentioned Extremist and Not So Different from Magneto. Of course, fans are divided as to how well this works.
 * Possibly Harry Osborn in the Spider-Man movies.
 * This was done to Anakin Skywalker throughout the Star Wars prequels. Of course, since these came out after the original trilogy, this ended up being a surprise to no one.

Literature

 * The Warhammer 40000: Eisenhorn Trilogy uses this on purpose, to show how an Inquisitor can go from Puritan to Radical in the course of duty. It displays the slide as a series of decisions that gradually get more and more radical as time goes on (ie )
 * Eisenhorn isn't really an example of a Villain Protagonist, as all through the series his motive was to protect innocent people (particually those around him) and this didn't change a bit. Eisenhorn didn't change his motives but his perspective on Imperial dogma did change. The  action that caused his death was  . Hardly an evil action.
 * In the Dresden Files books,.
 * Delivered somewhat in Harry Potter (it turns out not quite as outright villainy, but as  and  ) via a surprising instance of Posthumous Character.

Live Action TV

 * The progatonist from "The Brain That Wouldn't Die" (as seen on TV's Mystery Science Theater 3000) is also the main villain. We're not supposed to notice at first, but given his first few lines of dialogue it's rather like staring at the sun.
 * The put-upon lab assistant in this film is given a similar treatment; he seems almost sympathetic until, suddenly and quite inexplicably, he cops a nasty attitude halfway through the movie, just in time for his Karmic Death.
 * Angel, in Season Two
 * And again after they take over Wolfram & Hart.
 * Based on the most recent episode, Henry from Eureka
 * Averted in that he didn't do anything truly villainous, and continued to stay the same friendly face he had been. Truthfully, all he did was take more of a Darker and Edgier turn.
 * Lex Luthor has undergone this treatment in Smallville. Of course, we already know where that one is headed.
 * As of S9, Chloe Sullivan seems like she's on that path as well.
 * Peter from Degrassi the Next Generation is in and out of the Villain Corner like a grade-school dunce. In his first appearance, he's a blackmailer; afterward the show desperately tries to make him seem like a good guy (including mandatory Freudian Excuse). But then he has several more Kick the Dog moments, most notably when he helps a pedophile stalk somebody... and then an additional season of "no, he's really a decent guy." And after that, (judging by promos) it looked like he was going to be evil again. But as it turns out, you can Never Trust a Trailer, and after one episode of appearing to be evil it turns out he's still good.
 * In Supernatural S4, one common complaint was that it was impossible to empathize with Sam for  because not enough was done to reveal his motivation for it. Ultimately, Sam was   an example of this trope, and this point has been addressed in S5.
 * in the finale season of the Battlestar Galactica Reimagined remake. His motivations (and emotional breakdown) are made clearer if one has seen the webisodes.

Web Comics

 * In Order of the Stick, the wizard Vaarsuvius began showing signs of Jumping Off the Slippery Slope after the defeat at Azure City left him (her?) suffering from Bad Dreams over the failure of hir arcane power to save the day. It started with being a Jerkass to the rest of the party, progressed to, and finally to a full-blown that eventually led to a near  that very nearly pushed hir over the Moral Event Horizon. Fortunately, V shows signs of desiring to redeem hirself at the end of the arc.
 * The universe has made sure to give V another slap for the last action by revealing that
 * In Sluggy Freelance, while Dr. Schlock was always more concerned with saving his own hide than anything else, still seemed like a pretty sympathetic guy until he took over Hereti-Corp. After that, a storyline focusing on him was actually titled "A Year in the Life of a Villain."
 * In The Wotch, Miranda West has always been a Jerkass, but when she it's a pretty sinister act, even if it was delivered to a villain. Miranda's catapulting leap into the Villain Corner continued even further when