Is Macbeth a tyrant?

Macbeth does not start out as a tyrant, but he gradually becomes one over the course of the play's middle acts. When the play opens, Macbeth is an acclaimed military leader who performs valiantly in a battle and is awarded with the additional title Thane of Cawdor by King Duncan. After the battle, though, Macbeth meets the witches on the heath and learns that he is predicted to become king one day. He immediately starts to think about how he can become king quickly, and his ambition grows over the course of act I until he finally murders Duncan while he is visiting Macbeth's castle. Macbeth is crowned King. 
Once he is the monarch, Macbeth's ambition only increases; he is not satisfied with having been crowned, and he becomes completely paranoid about holding onto his position. His insecurity leads him to become increasingly tyrannical as the play progresses. He tries to eliminate all of his enemies, even those who were formerly his friends. He sees Banquo and Macduff as threats and tries to eliminate them and their families (he is partially successful). It is reported by other characters in the play that Scotland has declined radically under Macbeth's rule and that the people do not have food and live in fear of their king. When Macbeth is finally killed by Macduff in act V, he proclaims that Macbeth's decapitated head will stand as a monument to what happens to tyrants in Scotland.