Macbeth has many character flaws, but the one that is his downfall is his moral decline. As they say, power begets power, and Macbeth is an example of this. He convinces himself that he is the rightful heir to the throne, even though he has to kill the King to be so. At the start of the play, Macbeth is a hero, but he is easily swayed, shamed by his wife, he continues his downfall. His morals become less important to him, and power more important. Macbeth becomes evil because once he slips down that slope he slides all the way down and cannot claw his way back up to moral again.