Macbeth’s tragic flaw is his ambition. It leads to his downfall. He was a loyal and brave soldier before that, which is why Duncan promoted him to Thane of Cawdor in the first place. Macbeth becomes so overwhelmed with ambition, however, that he makes terrible decisions that lead to his destruction.
Macbeth’s ambition seems to be the direct result of the witches’ intervention. They tell him he will be king, and then he expects it. When he isn’t named Duncan’s successor, he gets angry. He announces to the audience in an aside that he has a dark desire to be king.
The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires (Act I, Scene 4).
Macbeth’s downfall is slow and messy. It begins before he even kills Duncan. He struggles with the process, imagining a floating dagger as he considers murder. The deed itself makes him imagine things, which causes him to lose sleep.
Macbeth’s descent into madness is also accompanied by paranoia. He has Banquo killed, both because he fears Banquo suspects Macbeth killed the king and because the other prophecy said Banquo’s sons would be kings. Macbeth then imagines or sees Banquo’s ghost. Macbeth also has Macduff’s family killed.
The witches return with a new set of prophecies, this time telling Macbeth to beware Macduff in one breath and that he can’t be harmed by a man “of woman born” in the other. Macbeth becomes even more unnerved when Lady Macbeth dies, appearing to commit suicide.
Macbeth is still not ready to give up. His ambition means he is ready to fight to the death, even if it means throwing his soldiers at a lost cause. When Macbeth finally faces Macduff, he is completely immolated by Macduff’s revelation that he was “from his mother's womb/ Untimely ripp'd” and therefore is a risk to Macbeth (Act V, Scene 8). Macbeth loses confidence in himself from that point on, making it pretty easy for Macduff to defeat him.