what makes Macbeth a tyrant?

Macbeth begins as a somewhat sympathetic character. He is loyal to the king, he is brave in battle, and is happy to win the king's praise. His ambition and his wife's ambition turn him into the tyrant that we know. The moment he kills Duncan sets of a chain of events that he can never come back from. Killing becomes easier and easier for Macbeth until he has lost all sense of right and wrong. Life becomes meaningless for him: even his own life becomes disposable at the end of the play.