Macbeth and his wife are initially convinced of the correctness, even righteousness, of their chosen path. They believe that Macbeth should be king and that he would be a better king than Duncan. For those reasons, they do not anticipate that they will suffer any negative feelings as a consequence of committing murder.
However, they could not be more wrong. Although the play is a political story about the change of rulers in Scotland, it is primarily an exploration of the mental deterioration that guilt causes in both Macbeth and Lady Macbeth. William Shakespeare uses the motif of “washing” throughout the play for expiation of the sin of guilt, represented by blood.
Even before he kills Duncan, Macbeth starts feeling guilty. He hallucinates a bloody dagger. As soon as he kills Duncan, Macbeth's guilt becomes a dominant emotion. He loses his powers of concentration and nearly exposes himself as the killer; Lady Macbeth has to remind him to clean up the blood. Because he actually did the killing, he feels remorse, but Lady Macbeth believes that "a little water clears us of this deed."
Macbeth decides to eliminate other rivals and potential threats, even children. After that, Lady Macbeth can see her husband coming unhinged, in part from guilt. Still, she considers herself free of such unproductive emotions. Soon, Macbeth can no longer sleep because it brings nightmares. He realizes that his deeds have created this situation: “Macbeth doth murder sleep. . . . Macbeth shall sleep no more.” A sleep disorder later overtakes his wife: she is constantly sleepwalking and miming washing her hands. The idea of the guilty stain that cannot be washed out is emphasized, as she tries to order it out and then poses a rhetorical question, “Out, damned spot. . . . [W]ill these hands ne'er be clean?” Guilt ultimately causes her to take her life.