Macbeth is a play that highlights this turmoil over the Scottish throne. When the play begins, King Duncan is at war with both Ireland and Norway. Scottish rebels, Macdonwald and the Thane of Cawdor, are helping these foreign forces in a fight against King Duncan. Macbeth is the hero in defeating these forces. Later Macbeth murders King Duncan and blames the killing on Malcolm, the king's son. In this way, Macbeth wrongfully becomes king. He becomes such a tyrant that Malcolm with the aid of the English invades Scotland and reclaims the throne.
Duncan acts as you would expect a king to act. When he discovers that the Thane of Cawdor has joined with Norway to overthrow him, he orders that the thane be executed. Because Macbeth has been loyal to the king and has been a great hero in battle, the king rewards him with the title of Thane of Cawdor, which was stripped from the traitor.
King Duncan is quick to punish his enemies and reward his supporters. Even though Macbeth has been instrumental in saving Duncan's throne from invaders and rebels, the king names his son Malcolm, not Macbeth, to be Prince of Cumberland and heirs to the throne. This is no surprise because the king's son is usually the heir to the throne. Further, the king is good to Macbeth. Not only does he name him Thane of Cawdor but he gives Lady Macbeth a diamond and gives other gifts of "great largess" when he visits Macbeth's castle. Duncan is shown to be the good king in this play.
Macbeth is a good and loyal supporter of King Duncan at first. He and Banquo are the heros of the battle against Ireland, Norway and the rebels. Macbeth has no thought of being king. After all, Duncan is king and has Malcolm as his son. It is only after the encounter with the weird sisters and their prophecy of his kingships that he yearns to be king. This is especially so after he obtains the title "Thane of Cawdor" just as had been foretold by the witches. Forced on by his overly ambitious and greedy wife Macbeth murders King Duncan and suspicion falls upon Malcolm for the deed. When Malcolm flees to England, Macbeth is next for the throne. Macbeth understands that his kingship is threatened by Banquo, because of the witches prophecy that Banquo's children will be kings. This forces him to murder Banquo and his son. Fleance, Banquo's son escapes, so the prophecy still can come true. So back goes Macbeth to the witches and four more prophecies are given: beware Macduff, Macbeth cannot be killed by a man born of a woman, Macbeth will not be defeated until the woods of Birnam go to Dunsinance hall and Banquo's descendants will be kings. These prophecies made Macbeth think that he is unbeatable but it is all for nothing because Banquo's heirs, not his, will be kings. This causes Macbeth to become harder, cruel, and a tyrant. He has become a very bad king.
In conclusion, in contrasting King Duncan and King Macbeth, King Duncan is the good king and Macbeth is the bad king. Duncan acts as a king should act while Macbeth is a murdering tyrant.