Lady Macbeth does stress that she would have killed King Duncan herself had he not resembled her father as he slept. This indicates that she a murderous heart. She could not bring herself to stab King Duncan only because he looked like her father as he lay sleeping.
The question is would she really have murdered King Duncan if he had not resembled her father as he lay sleeping? In her own words, she confesses murderous thoughts:
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(15)
My father as he slept, I had done't.
As Lady Macbeth claims that she would have killed King Duncan had he not resembled her own father, she is sharing an insight into her own character. The fact that she even admits that she would have murdered King Duncan is an indication that she has a murderous nature. As she confesses that she could have killed King Duncan, she only has one condition for not murdering him herself. King Duncan resembled her father as he lay sleeping. At least Lady Macbeth has some type of respect for her own father, but had King Duncan not resembled her father, she would have done the deed herself. Clearly, she has murderous thoughts in her heart and head. In the end, she convinces Macbeth to kill Duncan. She is as guilty as Macbeth is.