Alack, I am afraid they have awaked
And ‘tis not done. The attempt and not the deed
Confounds us. Hark! I laid their daggers ready.
He could not miss ‘em. Had he not resembled
My father as he slept, I had done’t (Act II, Scene 2).
This whole speech by Lady Macbeth is intended to "humanize" her. It shows that she is not nearly as confident as she has been acting with her husband. She is good about hiding her fears, worries, and guilt from other people, but she is still human. Shakespeare usually tried to show that all his characters were mixtures of good and bad, strength and weakness. That was why he showed the wicked King Claudius at prayer in Hamlet. It is the only way an author can create the illusion that his characters are real people.
When Shakespeare has Lady Macbeth say "Had he not resembled / My father as he slept, I had done’t," the purpose is consistent with everything else he has her say in this speech. It shows she has a tender side to her nature. She loved her father, as most daughters do. She is not a complete monster.