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.