A hideously frightening character, Lady Macbeth is an evil woman who is the main cause of Duncan's murder. At her first appearance in the play, she is already plotting Duncan's murder and is more ruthless and ambitious than her husband, Macbeth. Lady Macbeth remains fully aware of this and knows that she will have to push Macbeth into committing murder so he can claim the throne. At one point, she wishes that she were not a woman so that she could do it herself, seeing how Macbeth is insecure and questions the decision to murder Duncan. Lady Macbeth manipulates her husband with remarkable effectiveness, superseding all his doubts. Lady Macbeth repeatedly questions his manhood until he feels that he must commit murder to prove himself. Soon she says, "Come, you spirits that tend on mortal thoughts, unsex me here; and fill me, from the crown to the toe, top-full of direst cruelty! Make thick my blood" (Shakespeare).
Finally, after Macbeth murders Duncan, Lady Macbeth tries to sooth her husband's troubled spirit. Bitter, biting, and truly evil, Lady Macbeth is the main cause of the murder of Duncan.