19. Tyrion murders Shae
Season 4, Episode 10 "The Children"
When Tyrion got broken out of jail by none other than his big brother Jaime, he knew that there was one thing left to do before he fled Westeros for good. His father Tywin, who always belittled and abused him, needed his comeuppance -- doubly so because Tywin basically did nothing to help Tyrion while his phony trial was underway. Yes, fearsome Tywin dies by crossbow bolt while sitting on the toilet, but it's what happens before this that's truly disturbing. When Tyrion enters Tywin's bedroom, he finds Shae, his lost love, who had testified against him at his trial, lying in Tywin's bed. In a moment of rage and grief (and, partially, self-preservation, since Shae does go for a knife) he jumps on top of her and strangles her with her own necklace.