First idea: Does Macbeth only feel guilt because his ambition was too great for his own talents and strengths? His plan worked and Duncan is dead. If he were the man he had believed he was, would be be guilty?
This is a way at looking at Macbeth's ambition as a self-deception and his guilt as a self-realization. He is guilty because he has fooled himself into a false self-concept...
Second idea: Can we read Macbeth as a greek style tragedy where guilt, moral punisment, and the pangs of conscience are divinely oriented? If so, when one's ambition is to committ an evil act guilt will surely follow.
This is a way of looking at the story as a cause and effect tale of self-punishment. By asking for power, Macbeth has also asked for punishment.