Although King Duncan plays a relatively small role in Shakespeare’s Macbeth, he has two important functions. First, his murder helps drive the action of the play. Second, he seems to represent the ideal king whereas Macbeth seems to represent the tyrant ruler.
Duncan is an older man with two grown sons, Malcolm and Donalbain, and some of his inner qualities are that he is just, gracious, and generous. He warmly praises and rewards Macbeth for winning a battle for him. Even Macbeth acknowledges that Duncan is universally loved when he is thinking about murdering the king: “Duncan/ Hath borne his faculties so meek, hath been/ so clear in his great office, that his virtues/ Will plead like angels.” (1.7.16-19).
As a king, Duncan has been a gentle and blameless ruler. Once Macbeth has become king, he is soon viewed as a cruel tyrant, which underlines the contrast between the two.
As mentioned in our Background information, a king was anointed by God according to the Elizabethan world picture, which meant that ...