Joffrey beheads Ned (Episode 9, "Baelor")
This is the big one. A single swing of the royal executioner's sword kills off the show's best-known actor, eliminates the story's main character, and chucks the rules of television right out the window, Jaime-style. Here we learn that the world of Westeros is even less forgiving than we thought, and that Game of Thrones is playing for keeps. No wonder people freaked the eff out about it. All thanks to that repulsive little shit Joffrey – excuse me, King Joffrey, ugh – who calls his assholish audible in defiance of his desperate fiancée Sansa and scheming mother Cersei alike, cementing his place in the pantheon of TV villains you just want to watch get smacked in the face repeatedly.