Macbeth is not difficult to identify with because for the most part, we never lose sight of his humanity. The audience knows what the other characters in the play, except Lady Macbeth, do not: that Macbeth has gone through an extreme period of self-doubt and soul-searching before deciding to go through with the murder of Duncan. In Act II, Scene 1, for example, Macbeth is paralyzed by fear and gripped by his conscience as he debates whether or not to go through with the act:
But in these cases
We still have judgement here, that we but teach
Bloody instructions, which being taught return
To plague the inventor. This even-handed justice
Commends the ingredients of our poison'd chalice
To our own lips.
Despite Macbeth's considerable brutality and willingness to commit horrible crimes, there is also always a lingering (if highly debatable) sense that he is not single-handedly responsible for them. Both the witches and his wife act in different ways to push Macbeth toward a course of action that seems inconsistent with the esteem in which he is held early in the play. In short, power and ambition corrupt, and the audience may sense that they could be victims of these forces just as much as the protagonist. Macbeth is indisputably a villian, but he is a complex one, which is one factor that helps account for the play's lingering relevance.