Macbeth's major tragic flaw is his ambition, as he himself reveals in his soliloquy in Act I, Scene 7:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other—
It is Macbeth's ambition that causes him to kill the King, an act that leads to his downfall. But Macbeth is also very trusting and easily persuaded. It never dawns on him until the climactic final scene that the witches might be misleading him by telling him he is essentially invincible. They are telling him what he wants to hear, and he never really questions them--indeed, he bases all of his actions on their prophecies. Lady Macbeth also easily persuades her husband to carry out the murder of the king, even when he has, after careful consideration, decided to put it off for awhile. Macbeth, while a very powerful man, allows himself to be led by others. That these "others" were female would have suggested to Shakespeare's audiences a weakness of will that they would have seen as an inversion of the natural order.