Why does Lady Macbeth not kill Duncan herself in Macbeth?

The answer you are looking for can be found in Act II
scene 2 of this great tragedy, which is of course the scene during which the murder
takes place. Interestingly, however, Shakespeare chooses to focus on the action outside
of the room where Duncan sleeps, and so we are able to see Macbeth and Lady Macbeth
before and after the murder and see how they are psychologically impacted by the
crime.

What is interesting to note is that Lady Macbeth,
who up until this point has been so resolute and the one who has had to persuade her
husband to commit the crime, now shows one moment of weakness. She herself tells us that
she would have killed Duncan herself, but for the way that she was struck by his
resemblance to her own father as he lay there
sleeping:

Had
he not resembled

My father as he slept, I had
done't.

Thus we see that Lady
Macbeth is not necessarily as evil and as lacking in compassion as we would guess from
her behaviour and speech up until this point in the play.