Moral Event Horizon/Playing With

Basic Trope: A character (usually a villain) performs a vile deed which causes the audience to no longer sympathize with him/her.
 * Straight: The villain mutilates and murders innocent people out of boredom.
 * The villain kills one of the main characters.
 * Exaggerated: The villain mutilates random people he comes across on a whim and whenever he feels like it.
 * Downplayed: Kick the Dog
 * Justified: The vile deed was perfectly pragmatic, and the villain doesn't care what others think of him so long as they fear him.
 * Due to circumstances, the villain must do whatever it takes to accomplish his goal. Even if he comes off as inhuman, and requires abandoning his own code.
 * Inverted: The villain does something which endears him to the audience.
 * Alternately: It becomes a Crowning Moment of Awesome.
 * The act that would normally mark him as irredeemable leads him to realize just how callous he'd become and make an effort to be nicer than before. Often followed immediately by a Heel Face Turn.
 * He committed a petty crime.
 * Subverted: Turns out the villain was forced to do it.
 * Alternately: He was under Mind Control.
 * He does something later on that makes the aforementioned Moral Event Horizon a Kick the Dog moment.
 * Doubly Subverted: Turns out the villain was planning on doing it anyway.
 * Alternately: He had himself mind controlled for deniability.
 * It was All Just a Dream; the "Kick the Dog" moment was a Moral Event Horizon after all.
 * Parodied: All the villain does is Poke the Poodle, but the rest of the cast treats it as the most despicable action ever committed.
 * The villain strangles one of his own minions with his own intestines because he spilled his coffee.
 * Deconstructed: The villain becomes so repulsive his own followers rebel and kill him.
 * The villain did such a despicable thing, because he wanted to be hated and struck down by the heroes.
 * The villain has done such a cruel action, that the only people he cared for leaves him.
 * Reconstructed: The villain only employs people who are as vile as he is, or else are too frightened of him to revolt even over the evilest of acts.
 * However, heroes make the case that the villains motivations for wanting to get slain by them are ambiguous and thus dubious. He could as well have wanted his name immortalized by having fought and died struggling against them. His deed thus still deserves a more fitting punishment.
 * Only the hero and the audience see the villain crossing the Moral Event Horizon; he remains a Villain with Good Publicity to everyone else.
 * Zig Zagged: The villain mutilates and murders a major character. However, (much) later provides a good reason for it - except it turns out to be a lie. The villain seems to loathe himself for doing it, but heroes have reasons to suspect it's more about selfish reasons. Later the real reason, which is actually a good one, is discovered. Few are convinced, but the villain is still sympathetic.
 * Averted: The villain remains sympathetic throughout.
 * Enforced: "OH SHIT! The villain is experiencing Villain Decay. We have to make him vile and threatening again! Let's have him break the necks of some puppies and slaughter the hero's entire village!"
 * Lampshaded: "I have no doubt this cost me the last of my sympathizers."
 * "You're going to Hell for that."
 * "You've gone too far this time!"
 * Invoked: The villain gradually loses his sanity and becomes more inhuman over the course of the story.
 * Defied: Even Evil Has Standards
 * Discussed: "That evil bastard! I'll never forgive him for what he just pulled!"
 * Conversed: "That evil bastard! I hope he dies before the season's out!"
 * Played for Laughs: He murders beetles instead of people, and the heroes treat every beetle killed as a Moral Event Horizon.
 * Played for Drama: The villain has murdered millions of children or other innocent things, and all because he wanted to.

Back to Moral Event Horizon. And don't you ever think you will regain our sympathy when you get back there, You Bastard!