After the murder of King Duncan, Lady Macbeth continues much as she did before the murder. She appears cold and uncompassionate, while Macbeth is stunned, nearly in shock from what he has done. When Macbeth comments on what a "sorry sight" Duncan is, she says that this is "a foolish thought." She then goes on to tell him to "consider it not so deeply"; in other words, not to give it too much thought. Otherwise, she fears, "it will make [them] mad."
She then encourages Macbeth to return to the scene of the crime to leave the grooms' daggers and smear their faces with Duncan's blood so that they will appear guilty. Macbeth refuses, claiming that he can't look again at what he has done. Lady Macbeth then does this herself, showing how completely she is ignoring her pangs of conscience. That her conscience is, in fact, punishing her is made clear by the way that she psychologically unravels later in the play.
The final ironic divide between Lady Macbeth's attitude and that of her husband occurs later in the scene, lines 59-62, where Macbeth claims that the blood on his hand can never be cleaned off, rather, it would turn the entire ocean red. After he says this, Lady Macbeth enters and begins to wash her hands, announcing that "a little water clears us of this deed." She has completely ignored her humanity to accomplish her goal.
Lady Macbeth appears to be completely unaffected by what has been done. She is not at all concerned or bothered by the enormity of the deed that her husband has just committed. She and her husband have plotted and executed the murder of Duncan, who is a guest under their protection, a relation, and the king. This malicious act exceeds the bounds of morality; their deed has overturned the natural order.
When Macbeth returns from Duncan's chambers after having assassinated him, he is clearly upset and most perturbed by what he has done and experienced. He is shaken and acutely aware of his surroundings. Lady Macbeth, however, is cool, calm, and collected.
When Macbeth looks at his hands, which are tainted with Duncan's blood, he declares that they look pitiful. He is clearly regretful of what he has done. Lady Macbeth, though, tells him not to say such a foolish thing. She seems to lack remorse and does not seem bothered at all. Macbeth's remarks about the two chamberlains in Duncan's room who prayed and the fact that he could not say "amen" shows his deep distress. His wife, conversely, tells him not to consider what transpired "too deeply."
It is obvious that Macbeth is losing his hold, while his wife is maintaining her composure. He is unable to really fathom what he has done and displays deep remorse for his malicious act. He states his guilt by declaring that he heard a voice cry out:
"Glamis hath murder'd sleep, and therefore Cawdor
Shall sleep no more; Macbeth shall sleep no more."
His wife tells him that his imagination is running wild—the result of a sick brain. She urges Macbeth not to think in such terms. She then calmly tells him to wash the blood off his hands. She very deliberately gives him practical advice and asks him to return the bloodied daggers which he has, in his anxious and shocked state, brought down from Duncan's chamber. He should then smear Duncan's blood on the grooms to implicate them. The depth of her evil cunning is clearly on display here.
Macbeth refuses to go back to Duncan's chamber and says that he is afraid of what he has done and cannot bear to look at the bloody scene again. Lady Macbeth responds by telling him that he is "infirm of purpose." She is essentially saying that he is a coward. She decides to take the daggers back herself and smear Duncan's blood on the grooms' faces.
Macbeth is still quite anxious and nervous about what has transpired and is frightened when he hears knocking. He looks at his hands and believes that all the oceans of the world could not remove Duncan's blood from them; his hands would rather turn the oceans themselves red.
Lady Macbeth, at the end of the scene, expresses her shame for her husband's cowardly behavior and tells him to pull himself together; they need to get on their nightgowns if they should be called upon. It is ironic that Lady Macbeth should display such ruthless resolve here for, at the close of the play, she is the one who cannot cope with her guilt and commits suicide.