Judging from Macbeth's and Lady Macbeth's behavior and its psychological consequences, Shakespeare understood the human psyche long before Freud analyzed it and modern psychiatry categorized the ways it can come unglued. The Macbeths’ killing Duncan to steal the crown illustrates clearly the interaction of the id, the ego, and the super ego, as Freud identified them, and in the couple’s subsequent psychological deterioration, several manifestations of mental illness are evident.

The complexity in the Macbeths’ characters, and a great deal of the play’s drama, is developed through the interplay among these elements of the psyche. As psychologist Kendra Cherry explains, the id “strives for immediate gratification of all desires, wants, and needs.” After the witches' prophesy that Macbeth will be king one day, he and Lady Macbeth want the crown, and they waste no time in murdering Duncan to get it—the id at work.

After deciding to kill the king, the Macbeths are controlled by the ego, the part of the psyche that puts the brakes on the primitive id. The ego demands they satisfy their desires in a way that is acceptable to society. Initially, Macbeth rejects the idea of murdering Duncan, willing to delay gratification by waiting for the witches’ prophecy to be fulfilled without his intervention. Lady Macbeth, however, will not deny her need for immediate gratification. Through psychological manipulation in which she belittles her husband’s character and courage, Lady Macbeth forces Macbeth to proceed. There is no way to kill the beloved Duncan that will not outrage society, but through deceit, the Macbeths can make their behavior appear acceptable. They commit the murder surreptitiously with great attention to detail and skillfully deflect suspicion from themselves when the king’s body is discovered. With Malcolm and Donalbain blamed for their father’s murder, the Macbeths claim the throne, but they cannot enjoy the gratification of their desires because the super ego, the repository of the conscience, asserts itself.

The super ego is obvious in Macbeth even before the murder. Macbeth hungers for the crown, but the thought of killing Duncan, his king, benefactor, and friend, appalls him. Before entering the sleeping Duncan’s chamber, Macbeth agonizes over what he is about to do; after committing the murder, he is tortured by his conscience. Unable to sleep, he obsesses about the horrendous nature of his crime. In Lady Macbeth, the super ego is submerged temporarily through an act of will; before the murder, she steels herself against it, mistakenly assuming it is an expression of feminine weakness. After the murder, she denies its existence, insisting to Macbeth that “a little water clears us of this deed.” Washing Duncan’s blood from their hands, she believes, is all that’s necessary to escape retribution. She fails to recognize the presence and power of the super ego. Ultimately, however, it overrides Lady Macbeth’s psychological defenses, and she commits suicide.

Shakespeare doesn't employ the vocabulary of mental illness, but the manifestations of depression and psychosis are evident in the Macbeths’ mental and emotional deterioration. Shakespeare also understands their source—relentless, unalleviated guilt and fear. Macbeth lives in fear from the moment he first consents to murder the king, and after killing Duncan, he is consumed with guilt and plagued with insomnia. At various times, he suffers from visual and auditory hallucinations, and as his condition deteriorates, he becomes increasingly paranoid. Macbeth feels hopeless, his despair and bitterness expressed most vividly in the play’s conclusion as he prepares to go to war.

Shakespeare devotes much of the play to detailing Macbeth’s psychological deterioration, but he illustrates Lady Macbeth’s in one scene. As she walks in her sleep, sighing pitifully and trying to wash her hands of blood that now exists only in her unconscious mind, the extent of her mental destruction is made clear. That she soon kills herself, as her servants feared she would, comes as no surprise. Her death is entirely consistent with the psychological unraveling of her personality.