Lady Macbeth states that she would have killed Duncan, but he resembled her father as he lay sleeping. This could have been an excuse, but this the excuse she used. The following words of Lady Macbeth reveal her inner thoughts. She appears to be impatient. She is nervous and fears the murderous deed has not been committed before they (chamberlains) awake:
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.
Clearly, she is as much of a cold-blooded murderer as Macbeth. She could have killed Duncan had he not resembled her father. At least she has some sense of decency in that she thinks of her father and cannot kill Duncan. Nevertheless, she is as guilty as Macbeth for encouraging him to commit the murder.
This question has already been asked and answered here on eNotes. Here is a link for you: http://www.enotes.com/macbeth/q-and-a/what-excuse-does-lady-macbeth-give-not-killing-251691