Final answer:
Lady Macbeth could not kill Duncan herself because he resembled her father as he slept, which emotionally prevented her from committing the murder.
Step-by-step explanation:
The reason Lady Macbeth couldn't kill Duncan herself is mentioned directly in the text of Shakespeare's Macbeth. In Act 2, Scene 2, Lady Macbeth reveals her own reluctance to commit the murder when she says, "Had he not resembled / My father as he slept, I had done't." This signifies that Lady Macbeth's decision not to kill Duncan was not due to physical incapability, fear of consequences, or believing it was solely Macbeth's role, but rather an emotional inability to harm Duncan who, in his sleep, reminded her of her father.