The short answer to this question is that Macbeth is a different character at the end of the play than at the beginning. He is portrayed early in the play as an honorable, brave lord and vassal, who is nevertheless fierce in battle. But his encounter with the witches immediately sparks his ambition, and he begins to contemplate horrible deeds. Yet he is very reluctant to kill Duncan, and shows remorse for it. By the end of the play, however, with several more deaths to his name, he is a murderous villain, unmoved even by news of his formerly beloved wife's death by suicide. His ambition, combined with fate and the supernatural, has destroyed whatever humanity there was in him.