The prophecy was a self fulfilling prophecy, MacBeth and Lady MacBeth only acted in the first place because of what the witches had foretold, he only acted because he thought that it was destined. Because of MacBeth's belief in the prophecy and in fate, he killed the King, whereas if he was not foolish enough to do as the prophecy spoke of then he never would have become a murderer.
I hope this hasn't confused you.