Can you kill youself in World of Warcraft

Not if you mean by some command line, but there certainly are abilities that could kill you, such as the Warlock's Hellfire. Hunters can Feign Death to appear dead. When you are at low health, a Shadowpriest's Shadow Word: Death can kill himself.

On the whole, there are abilities that damage you, but nothing that instantly kills you. The one ability that could do that (Divine Intervention from the Paladin) has been removed since just before the start of Cataclysm.