Macbeth wants to be king just to take power. He seems to do absolutely nothing kingly once he is there. A true king is a leader and cares about his country and his people. Macbeth clearly does not. He is more interested in himself. After becoming king, he kills many of his subjects to ensure that he stays king. He also leads them into pointless battle and risks their lives to maintain his tenuous throne.
When a servant tries to tell Macbeth that soldiers are coming, he does not want to hear it and threatens the servant, calling him a coward.
Go prick thy face and over-red thy fear,
Thou lily-liver'd boy. What soldiers, patch?
Death of thy soul! (Act 5, Scene 3, p. 80)
A true king would have taken stock in the situation and actually tried to protect his men.
Oedipus, on the other hand, is at least interested in helping his people. He tries to find out what is causing the plague, and once he knows he tries to act on it.
O Tiresias, who grasp all things,(315)
even if you cannot see, you still understand
what sickness plagues our city, and we find, lord,
you alone are our savior and defender.(p. 12)
Oedipus also cares about his family, and in the end he weeps for them. So while Oedipus does make mistakes, he seems to have a heart. Macbeth does not ever do anything to benefit anyone but himself. He is in the end the more selfish.