In Macbeth, ambition conspires with unholy forces to commit evil deeds which, in their turn, generate fear, guilt and still more horrible crimes. Above all, Macbeth is a character study in which not one, but two protagonists (the title character and Lady Macbeth) respond individually and jointly to the psychological burden of their sins. In the course of the play, Macbeth repeatedly misinterprets the guilt that he suffers as being simply a matter of fear. His characteristic way of dealing with his guilt is to face it directly by committing still more misdeeds, and this, of course, only generates further madness. By contrast, Lady Macbeth is fully aware of the difference between fear and guilt, and she attempts to prevent pangs of guilt by first denying her own sense of conscience and then by focusing her attention upon the management of Macbeth's guilt. In the scene which occurs immediately after Duncan's death, Lady Macbeth orders her husband to get some water "and wash this filthy witness from your hand" (II.i.43-44). He rejects her suggestion, crying out, "What hands are here. Ha! they pluck out mine eyes! / Will all great Neptune's ocean wash this blood / Clean from my hand?" (II.i.56-58). But she in turn insists that the tell-tale signs of his crime cannot be seen by others, that "a little water clears us of this deed" (II.i.64). But midway through the play, Lady Macbeth loses both her influence over her husband and the ability to repress her own conscience. Once her husband has departed to combat against Macduff's forces and Lady Macbeth is left alone, she assumes the very manifestations of guilt that have been associated with Macbeth, insomnia and hallucinations, in even more extreme form.
As for the motive behind the theme of guilt, it is ambition for power, and it does not require much for Macbeth to embrace the weird sisters' vision of him as the ruler of all Scotland. Macbeth is ambitious, but it is Lady Macbeth who is the driving force behind their blood-stained rise to the throne(s) of Scotland. Lady Macbeth is awesome in her ambition and possesses a capacity for deceit that Shakespeare often uses as a trait of his evil female characters. Thus, when she greets her prospective victim in Act I, she "humbly" tells King Duncan that she has eagerly awaited his arrival and that her preparations for it are "in every point twice done, and then double done" (l.vi.14-18). The irony here is that double-dealing and falsity are at hand, and Lady Macbeth's ability to conceal her intentions while at the same time making hidden reference to them has a startling effect upon us.
Beyond the evil that human ambition can manufacture, Macbeth has a super-natural dimension to it; indeed, the play opens with the three witches stirring the plot forward. Even before his encounter with the three witches, Macbeth finds himself in an unnatural dramatic world on the "foul and fair" day of the battle (I.iii.39). Things are not what they seem. After his first conclave with the witches, Macbeth is unable to determine whether the prophecy of the witches bodes "ill" or "good." He then begins to doubt reality itself as he states that "nothing is / But what it is not" (I.iii.141-142). The prophecy, of course, is true in the first sense but not what Macbeth takes it to be in the second. In like manner, the three predictions made to Macbeth in the first scene of Act IV seem to make him invincible; but the "woods" do march and Macbeth is slain by a man not ("naturally") born of woman.
Not only does an unnatural world overturn reality in Macbeth's experience, in Lady Macbeth's experience, this movement beyond nature is self-invoked. In an oft-cited speech, Lady Macbeth actively conjures up supernatural forces to change her into a creature without conscience or human (or "feminine") compassion.
Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me, from the crown to the toe topful
Of direst cruelty! Make thick my blood,
Stop up th' access and passage...
(The entire section is 2,176 words.)