Macbeth is a political play in that it is an exploration of the difference between good kingship and bad. In it, Shakespeare illustrates what makes a king a good ruler and what makes him a poor ruler. Unlike Machiavelli, Shakespeare comes down on the side of the personal character or morals—the virtue—by which the king lives as the definitive attribute of the good monarch.
The regicide which starts Macbeth on his reign makes it impossible for him to be a good king in Shakespeare's eyes, because it forces him down an immoral path. He must be always looking over his shoulder fearfully—fearful that he be betrayed by someone who suspects the truth, and moreover, fearful that a rival will do to him what he did to Duncan. As a result of the original evil path, Macbeth becomes more and more of a tyrant, putting his own needs ahead of those of his people and increasingly bent on reckless bloodshed.
In contrast, Malcolm, Duncan's eldest son, represents the good king. He is "meek," by which Shakespeare means gentle and good to those beneath him, he is honest, and he does not give in to his passions and lusts. He is self-controlled and puts the needs of his people first. Another good king is England's Edward, whose virtue allows him to heal the sick.
In Macbeth, Shakespeare makes a strong case for the prosperity of a country and its citizens depending on the justice, mercy, and overall moral virtue of its king.