What causes the downfall of Macbeth?
In general, Macbeth, in Shakespeare's play of the same name, suffers from hubris. He attempts to rise above his station in life by using unethical and immoral means.
Macbeth's hubris exhibits itself in the form of ambition. He is a highly decorated and respected thane, but that is not enough for him. He has to be king. To be king he assassinates Duncan. To eliminate a threat he orders the murder of Banquo. To enable his heirs to be kings--thus creating a dynasty--he orders the murder of Fleance. Out of spite, and because he cannot get to Macduff, he orders the slaughter of Macduff's family.
When Banquo's ghost (representing Macbeth's conscience) appears at the banquet and Macbeth twice loses control in front of his thanes, he does what his wife has been warning him about for much of the play: reveals his guilt to his supporters. He reveals that he is obviously unfit to rule. From that point on, Macbeth's fall is ensured.
Macbeth, in the conversation with his wife immediately after the stunned thanes exit, reverts to something he can control--his vendetta against Macduff. But going to see the witches and slaughtering Macduff's family can't save him. Macbeth is doomed.