Why would computers want to take over the world and kill all humans?

If Artificial Intelligence is possible, why would machines want to terminate humans? You see it all the time in sci/fi movies such as The Matrix or The Terminator. Machines wanting to exterminate all human life forms. Why would machines or computers have such an intention to do so? If they did do so, what would they do after they have killed off every living human creature? I really enjoy sci/fi movies, but I've always wondered about this.