Macbeth's wife is one of the most powerful female characters in literature. Unlike her husband, she lacks all humanity, as we see well in her opening scene, where she calls upon the "Spirits that tend on mortal thoughts" to deprive her of her feminine instinct to care. Her burning ambition to be queen is the single feature that Shakespeare developed far beyond that of her counterpart in the historical story he used as his source. Lady Macbeth persistently taunts her husband for his lack of courage, even though we know of his bloody deeds on the battlefield. But in public, she is able to act as the consummate hostess, enticing her victim, the king, into her castle. When she faints immediately after the murder of Duncan, the audience is left wondering whether this, too, is part of her act.
Ultimately, she fails the test of her own hardened ruthlessness. Having upbraided her husband one last time during the banquet (Act III, Scene 4), the pace of events becomes too much even for her: She becomes mentally deranged, a mere shadow of her former commanding self, gibbering in Act V, Scene 1 as she "confesses" her part in the murder. Her death is the event that causes Macbeth to ruminate for one last time on the nature of time and mortality in the speech "Tomorrow and tomorrow and tomorrow" (Act V, Scene 5).