In Shakespeare's play, Macbeth, Lady Macbeth suggests that she would kill Duncan herself, but something changes her mind—what is it?
In Shakespeare's Macbeth, Lady Macbeth is especially excited by the predictions the witches deliver to Macbeth while he is walking on the heath with Banquo. For if Macbeth becomes King, she will be Queen of Scotland, and Lady Macbeth's ambition equals Macbeth's, if it does not surpass it.
When Lady Macbeth learns that the King is coming to visit overnight, she greets her husband with her plans. She notes that while the King may arrive that day, he will never live to see the morning.
Shall sun that morrow see! (I.v.65-66)
Macbeth initially agrees with his wife's decision to murder the King, but as Duncan rewards him for all of his efforts on battlefield, Macbeth decides he might like to wait before acting. (Remember, also, that Duncan is not only his King, but his friend and his cousin.) Murder does not come easily to the noble man Macbeth is at the play's beginning. However, Lady Macbeth is incensed as he hesitates, and ridicules and insults her husband—along with his bravery and his manhood—until he agrees to proceed.
Once the King has gone to bed, part of Lady Macbeth's plan is to get his guards drunk; this leaves the sleeping King essentially protected. In Act Two, scene two, Lady Macbeth reveals not only that she has prepared the way for Macbeth's success (taking care of the "details," but that she would have done it herself if not for one reason:
Had he not resembled
My father as he slept, I had done't. (15-16)
Besides words of love spoken to her husband, this sentiment may be the only indication that Lady Macbeth has a feminine side to her at all. Only because the King looked like her father did Lady Macbeth not take the daggers and kill Duncan herself. As it is, when Macbeth cannot return the bloody daggers to the room where Duncan lies dead (because he is so shaken by his actions), Lady Macbeth does so, and smears the King's blood on the guards to provide incriminating evidence that it is they who have murdered the man they were supposed to protect.