Why can't Lady Macbeth kill Duncan herself?
At the beginning of Act 2, Scene 2, Lady Macbeth has evidently returned from Duncan's chamber. It is not clear whether she had been there before. She had to give Duncan's guards something to drug them and also lay out their daggers so that her husband could use them to kill Duncan. The idea is to plant the guards' blood-stained daggers on them and frame them for the murder. It is hard to see how Lady Macbeth could accomplish both tasks in one visit--drugging the guards and laying out their daggers for her husband. At any rate, she say Duncan sound asleep in his bed. She tells herself:
Had he not resembled
My father as he slept, I had done't.
She may or may not be rationalizing. After all, she has had no experience in killing people, whereas her husband has killed many. Why not leave the job to an expert? Her statement that she might have killed Duncan if he hadn't seemed to resemble her own father may be only Shakespeare's way of explaining why Lady Macbeth, who had the opportunity to commit the murder, didn't do it. Shakespeare may have intended to make this woman a more three-dimensional character. She is a woman, after all, and has a woman's feelings. She seems vicious and determined when talking to her husband, but when she is alone in Act 2, Scene 2, she reveals her doubts and fears.
Alack, I am afraid they have awaked,
And 'tis not done. Th' attempt, and not the deed,
Confounds us. Hark!
It would seem that Macbeth and his wife need each other. They give each other confidence. When they are separated, as they are at first in Act 2, Scene 2, they both become anxious and uncertain. It is to be observed that this is often the case with people who commit violent crimes, like the two young killers in Truman Capote's novel In Cold Blood. Separately these two young men would never have committed their horrible murders, but together they become a lethal pair, like two chemicals that are innocuous separately but volatile when mixed together. Macbeth would never have murdered Duncan if his wife hadn't pressured him to do it, and she needed her husband not only to bolster her nerve but to make the deed worth doing. She cannot become queen unless her husband becomes king.
Lady Macbeth had plenty of opportunity to kill King Duncan and didn't do it. Maybe it was because the old man reminded her of her father, and maybe that is only her excuse for not being able to commit such a bloody deed because she didn't have enough nerve.