I would have to say Tigerstar. I know it is predictable but he tried to kill Bluestar! He killed Firestar, Brindleface, Runningwind, he caused Swiftpaw's death and he made cats kill Stonefur. He caused the war in The Last Hope. He made Hawkfrost evil and tried to do the same to Brambleclaw. That cat is just plain evil! ~Spottedpool