Could Lady Macbeth's previous psychosis in response to an event in her past involving her father? From Act 2, Scene 2, Lines 11-12, Lady Macbeth says, "Had he not resembled my father as he slept, I had done 't" We already know she can't be perfectly stable mentally, but this could very well be the previous cause of her psychosis. Smith said Lady Macbeth goes insane later in the story, so it would easily explain why if she tied king Duncan to her father, at least in this particular situation.
I agree! I also think that it is ironic that Lady Macbeth encourages Macbeth to murder Duncan, but she won't even do it herself because she says he looks too much like her father. My question is, if she can't kill Duncan because he looks too much like her father, why is she alright with someone else killing him?