Macbeth can be viewed as a play in which a question is posed regarding what motivates men and women to carry out actions that are morally wrong and, in their ultimate effect, self-defeating. Many would say that there is really no definitive answer; as most authors do, Shakespeare presents his themes in such a way that the underlying or implied "message" is open to interpretation, and different readers and audience members may reach opposite conclusions.
On the surface, Macbeth is motivated by a lust for power. He wants to be king, and it is apparent that, given his reaction to the witches's prophecy, he has probably already contemplated some step in which he can eliminate Duncan and take his place. But there is a mystery at the root of his actions. Unlike Lady Macbeth, who seems to have no compunctions about urging her husband to commit murder, Macbeth does not want to kill Duncan:
We will proceed no further in this business. He hath honoured me of late, and I have bought Golden opinions from all sorts of people, Which must be worn now in their newest gloss And not cast aside so soon.
It takes only a couple dozen lines of persuasion from Lady Macbeth to make him change his mind. But why? Already, Macbeth is suffering, hating himself for wishing to take this step. The "air-drawn dagger," the "fatal vision" he hallucinates (or is it real?)—which Lady Macbeth later refers to, along with the ghost of Banquo, as "the very painting of [his] fear"—is a symbol of Macbeth's guilt, of the fact he knows that in committing himself to this course, he is destroying his own life as well as that of the king and, later, the lives of Banquo and others.
What does this tell us about human motivation? Is the overall message that we are moved to do things we acknowledge are morally wrong and self-destructive? Macbeth, like other Shakespearean characters, is a kind of abstraction—a grand-scale representative of what all people are, with our own more petty and less violent thoughts and actions. Is the subtext that motivation is an unknown—that, put simply, we don't understand why we do things? Throughout the play, Macbeth acts as if he's in a trance, seemingly without the ability to stop himself from committing crime after crime. He is not a sociopath, a person without any moral sense who kills without remorse. That he suffers as he does is the reason the play is so powerful. Questions are also raised often about the motivation of the witches: are they moved by some unknown force to create havoc through their prophecies? Or are they not intended to be seen as actual living beings, but rather a symbol of the underlying mystery, a primal force that causes real people's motives to be irrational and self-defeating?
The key may lie in what are perhaps the most quoted lines in Macbeth:
Life's but a walking shadow, a poor player That struts and frets his hour upon the stage And then is heard no more. It is a tale Told by an idiot, full of sound and fury, Signifying nothing.
Macbeth realizes that all of it has been meaningless, yet, as mentioned, opposite conclusions could be drawn as to the "moral" of the play. Is it that man is responsible for his acts, or rather that we are all helpless victims, the puppets implied in Macbeth's words before he goes to his death?