Macbeth--Political Legitimacy?

One of the important themes in Macbeth is the idea of political legitimacy, of the moral authority that some kings posses and others lack. With particular attention to Malcolm's questioning of Macduff in Act 4, Scene 3, try to define some of the characteristics that grant or invalidate the moral legitimacy of absolute power. What makes Duncan a good king? What makes Macbeth a tyrant?