There are a few different stages to Macbeth's mental deterioration. In Act II, we see Macbeth nearly paralyzed with guilt over the murder of Duncan. We understand that, while he is ambitious, regicide was not his intended plan for gaining power. In Act III, we see the growth of his guilt. He thinks he sees Banquo at the dinner table, when he knows that Banquo is dead. He speaks to the ghost, not realizing at first that everyone cannot see him. Eventually, Lady Macbeth has to break up the dinner party, excusing her husband for being ill. In Act IV, out of desperation, Macbeth visits the witches again and demands to know more. Feeling empowered by their prophecies, he embraces battle with Malcolm. He feels like he is undefeatable, and using that feelings to let his hungry for power consume him, making him mad with power. Ultimately, he is forced to face reality in his battle with Macduff, which ends with his death.
William Shakespeare’s Macbeth is the story of a man whose fledgling ambition makes him susceptible to manipulation, which then grows into an insatiable appetite for power. If we want to look at how his mental health deteriorates, we have to look at what causes the problems that push Macbeth over the edge. Macbeth’s mental health is adversely affected because he is manipulated to act not in accordance with his own will, but with the will of others.
Initially, Macbeth is manipulated by the witch’s prophecy that he will be king. Although this was never his ambition, he begins to obsess over it, and tells his first lie to Banquo when he says of the witches and their prophecy:
I think not of them.
A little later, he is manipulated by Lady Macbeth, who convinces him to kill Duncan. At this point Macbeth’s mental health is reflected in the guilt he feels even before the murder, when his speaks of his vision of the dagger:
I see thee still;
And on thy blade and dudgeon gouts of blood,
Which was not so before.
Once Macbeth starts making decisions for himself, the audience can see that he has been transformed from a noble thane to a fiend. He kills Banquo and tries to kill his son; he kills the whole of Macduff’s family, children and all; he seeks out the counsel of the witches again and falls for their trap.
If Macbeth had followed his own instincts and not allowed himself to be duped by the witches and Lady Macbeth, his mental state would have been unaltered. By subjugating his will to that of someone else, he sacrifices his own mental health and his own destiny.