Macbeth's tragic flaw is his ambition. He is willing to do anything it takes to become the king, and his wife encourages his evil deeds.
When the play begins, Macbeth is one of the king's generals. He meets three witches who predict that he will become Thane or Lord of Cawdor, and then king. The witches also predict that the heirs of a fellow general, Banquo, will be king after him.
Macbeth doesn't believe the prophecy until he is named Thane of Cawdor. He decides to kill the current king, Duncan, to help the prophecy come true. Macbeth takes the throne, but he becomes more and more afraid that Banquo's son or another enemy will take it away from him. He slowly descends into madness. Macbeth's fears come true when an opponent kills him.