What excuse did Lady Macbeth give for not killing Duncan in Macbeth?

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:

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LADY MACBETH 

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.