Shakespeare portrays King Duncan as a good and wise ruler. Macbeth describes him as "meek," a positive term meaning humble, and "so clear in his great office," meaning uncorrupted by his power. Duncan behaves well to Macbeth, and by making him Thane of Cawdor, Duncan shows that he is doing nothing to prevent Macbeth from rising in the world. In fact, Duncan is helping and mentoring Macbeth. He trusts Macbeth and his wife completely, and believes they will show him every hospitality when he comes to stay with them. This heightens the crime that Macbeth commits when he murders Duncan, for this king is no tyrant or despot. Macbeth knows from the start that there is no rationalization he can hide behind in killing him, and that Duncan's strong, virtuous character will
plead like angels, trumpet-tongued, against
The deep damnation of his taking off.
In other words, Duncan is so good that his moral goodness will increase Macbeth's damnation for killing him.
In fact, Macbeth gets cold feet and wants to back out when he remembers what a good king Duncan has been. It takes Lady Macbeth to steel him to the task by saying he promised to kill Duncan. She knows that Macbeth's loyalty to Duncan is strong, so she says she would have dashed her own baby's brains out if she had promised to do so. She seems to realize that she has to speak in such strong terms or Macbeth won't act.
Duncan's goodness highlights that it is solely ambition that drives Macbeth. He has no other reason to want to kill his ruler.
King Duncan in Macbeth by William Shakespeare is first of all a legitimate ruler rather than a usurper. In this period, when kings were considered rulers by divine right, Duncan appears to have all the qualities associated with kingliness. His demeanor is polite, gracious, and regal. He is elderly, but wise, and has a fitting sense of occasion.
He appears to be a good leader, willing to reward his followers when they do outstanding work in his service. He also has a strong sense of justice, dealing harshly with rebels and treating people who are loyal to him well. He also seems kind and trusting towards those who are in his service. Even as Macbeth and Lady Macbeth plot to kill him, they acknowledge that he is a good king and they have no complaint about him as a leader; he merely stands as an obstacle to their plans.