How do Macbeth and Duncan exemplify the qualities of a good king in Macbeth?
This is a very interesting question, and the key to answering it, I would suspect, is to first define what a good king actually entails. It's tricky, because different generations have different answers to this questions (times have changed a lot since Shakespeare's day. Especially in the modern era, by which time, in the western world at least, monarchy has largely declined to a ceremonial position).
In any case, I'd suggest that good monarchs act to ensure the stability and security of the State. Whether its a Constitutional Monarch or an Absolutist Monarch, this has remain a central theme of Kingship. Therefore, I'd suggest that any qualities a monarch might possess have to be viewed within that context.
With this in mind, I don't think there is any quality which would qualify Macbeth for being a good King. Quite on the contrary, I'd suggest that Macbeth is first and foremost a usurper. He wins the throne through murder and proceeds to plunge the realm into unrest. He is cunning, and he's a capable fighter and commander, but as a King, I think his short lived, brutal reign speaks for itself. He throws Scotland into chaos and is then overthrown by the righteous claimant he usurped. He's a tyrant.
Duncan, on the other hand, I would say passes the test of being a good King. In the beginning of the play, we see Duncan's forces defeating an opposing army in the field, and Duncan follows this victory by rewarding his loyal vassals, naming Macbeth the Thane of Cawdor. At the same time, he names his son, Malcolm, his heir, thus ensuring (what should have been) a stable succession. All of these decisions would have had the intention of further ensuring the stability of his dynasty and stability within the Realm.
Duncan is a good king in that he rewarded loyalty. Macbeth was a good king because he was decisive. However, each king also had bad qualities.
Duncan was a good king because he gave Macbeth a promotion to recognize his bravery and loyalty on the battlefield. He even feels bad when he does not honor him sooner.
O worthiest cousin!
The sin of my ingratitude even now
Was heavy on me. (Act 1, Scene 4, p. 17)
However, Duncan should have been more aware of what was going on. Macbeth later had spies to make sure everyone was loyal to him. Duncan could have used spies, or at least better guards. He was totally taken by surprise. He thought Macbeth would be thrilled at his promotion, and he did not see the betrayal coming.
Macbeth is described as a bloody tyrant, so he was far from an exemplary king. He did do some things well, however. He had a banquet for his supporters. He acted decisively when threatened.
To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be fear'd. ’ (Act 3, Scene 1, p. 42)
Of course, Macbeth takes it a little too far and is soon sending murderers to kill anyone he suspects is not faithful. You can’t run a kingdom that way. Macbeth gets the reputation of being a bloody tyrant, and Malcolm and Macduff come to unseat him.
Both characters exemplify various qualities of a good king in Shakespeare's classic play Macbeth. King Duncan exemplifies the qualities of benevolence, respect, and charity to his subjects. Following Macbeth and Banquo's victories, King Duncan awards both men for their valor, loyalty, and courage. King Duncan gives Macbeth the title of Thane of Cawdor and presents his wife with a diamond ring. Duncan also honors Banquo and informs Malcolm that he will become his heir. King Duncan also expresses his gratitude and appreciation for Macbeth and his wife when he visits Inverness. Although Duncan is kind, compassionate, and benevolent, he is rather naive and gullible.
Even though Macbeth develops into a bloodthirsty tyrant who has no qualms about murdering his political opponents, he has several qualities of a good king. Macbeth is both resolute and determined. He does not waver in his thoughts and takes immediate action when he is presented with an important decision. Whenever Macbeth acts, he does so decisively and is determined to make his thoughts a reality.