There are a few reasons why Lady Macbeth does not kill King Duncan herself. First, in Act 2, Lady Macbeth says that the king resembles her own father, so therefore she cannot kill him. More importantly, however, Lady Macbeth as a woman must follow the social conventions of her time. Earlier in the play, she prays to the spirits and asks that they "unsex" her so that she might have enough cruelty to help Macbeth go through with the murder. As a woman, she is supposed to be innocent and good-natured, so obviously committing murder does not fall into this persona. When Duncan is found dead by Macduff, Lady Macbeth faints and has to be carried out of the room. This is ironic because she is certainly aware that the king is dead; she must faint in order to play the part of the "gentle lady" that is expected of her.