King Duncan of Scotland
Duncan is a mature and effective king who rules Scotland during a time of political upheaval and turmoil. He is too old to command the armies himself and his sons are too young and inexperienced to fight in wars, but he has several great captains under his command, which allows him to remain in power. Those that remain loyal to him do so because they see that his rule is divinely sanctioned. When Macbeth hesitates in his decision to assassinate Duncan, the reason is that he fears Duncan's "virtues / Will plead like angels, trumpet-tongued against the deep damnation of his taking-off" (I.vii.17-18) - that the very heavens will object to the murder of a king so holy. It turns out that Macbeth's fears are justified - on the night of his murder, chimneys are blown down, the earth shakes, owls scream and kill falcons, and horses run wild and eat each other.
The elder son of King Duncan, Malcolm is named the Prince of Cumberland and heir apparent early on in the play. After his father is murdered at Macbeth's castle, he and his younger brother, Donalbain, decide to split up and flee for safety because they will probably be the next targets of the murderers. Although they are safe abroad (Malcolm in England, Donalbain in Ireland), they are not at home to defend themselves against accusations by the Scottish thanes (the equivalent of English earls), that they murdered their own father for the crown. In time, it becomes clear that Macbeth perpetrated the regicide on Duncan, and Macduff seeks out Malcolm in the English court to return to Scotland to claim his birthright and rescue them from Macbeth's tyrannical rule. Malcolm has since been prospering in the English court of Edward the Confessor, a figure remarkably similar to Duncan, and he has learned much about politics and warfare. By the time Macduff comes looking for him, he has already enlisted the aid of Siward, Earl of Northumberland, who has provided him with an army, ten thousand strong. He leads the soldiers to victory over Macbeth, and it appears that he will reign brilliantly at play's end because he is a virtuous and an intelligent young man.
The Thane of Glamis, then Thane of Cawdor, and then King of Scotland as the witches prophesy. He is Thane of Glamis by birthright, and he achieves Thane of Cawdor through his unparalleled bravery and prowess in a recent war to squelch a rebellion led in part by the traitorous Thane of Cawdor. Before he received the official word of his promotion to Thane of Cawdor, three witches visited him and his friend Banquo and predicted that Macbeth would become Thane of Cawdor and then the King of Scotland thereafter. The witches predicted for Banquo that he would father a line of kings, although he would not be a king himself -the implication being that Macbeth will die leaving his throne empty for Banquo's issue. The truth of the first prediction that he will become Thane of Cawdor, makes him hungry for the fulfillment of the second prediction, the throne of Scotland. At first, Macbeth is content to see what the fates have in store for them. After all, he might not have to do anything to achieve the throne since Thane of Cawdor was achieved so effortlessly. In a subsequent post-war meeting with Duncan and the other thanes, Duncan announces Malcolm as his heir. Macbeth realizes that the throne, on the other hand, will not come as easily. He tells his wife Lady Macbeth of the witches' predictions and their confirmed truth. He promises his wife royal greatness, and she makes him hold his promise. Even when it becomes clear to him that he should not commit the murder of Duncan for fear of the consequences, Lady Macbeth appeals to his impulsive and insecure side to proceed with the assassination. One could argue that were it not for the suggestion of the witches or the prodding of his wife, he never would have murdered Duncan, but he chooses of his own...