Clearly, the characters that most influence Macbeth in the play are female. Their roles remain the subject of debate among scholars, however, with some claiming that Lady Macbeth and the witches are more responsible for the events of the play, and others laying the blame at Macbeth's feet. The witches (and Hecate) exert a malevolent influence, largely by piquing his ambition with prophetic visions of his future. By telling him that he is to be king, the witches seem to realize that he will be tempted to murder his way to throne. By telling him, as the visions sent by Hecate do, that he can only be killed by someone not of woman born, and then only when Birnam Wood marches to Dunsinane, they give him a lethal overconfidence, as Hecate says:
He shall spurn fate, scorn death, and bear/His hopes ’bove wisdom, grace, and fear. And you all know security/Is mortals’ chiefest enemy.
If the witches nudge Macbeth along the path to self-destruction, his wife is the person who plots out the details of Duncan's murder, then steels her husband to commit the deed when he undergoes a brief moment of hesitation due to conscience. She challenges his manhood and his integrity, and ultimately prods him into the murder. This leaves open the question of whether Macbeth would have committed the murders at all without his wife's encouragement. An interesting aspect to the role of women in the play is that Shakespeare refers to them in androgynous terms that would have made them seem unnatural to his audiences. The witches, for instance, have beards, and Lady Macbeth asks for the "spirits that attend mortal thoughts" to "unsex" her, giving her the ruthlessness she needs to push her husband toward murder.