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.