If 'Macbeth' can be said to have a 'moral' I would suggest that the play principally shows how destructive ambition or over-ambition can be. Macbeth, who seems to be at heart a good man, is led into thoughts and actions that are fundamentally against his nature and lead in the end to the destruction of himself and others, including his wife, Lady Macbeth, who initially encourages him but quickly senses that he will not be able to hold on to what he has wickedly gained.