In Macbeth, why doesn't Lady Macbeth kill the king herself?

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.