These lines spoken by Lady Macbeth are from Act II, Scene 2. After Macbeth returns from murdering King Duncan, he carries the daggers with him; Lady Macbeth tells her husband that he must return the daggers and smear the blood onto the sleeping guards so that they will seem guilty of the murder. But, the shaken Macbeth, who has heard voices say, "Macbeth shall sleep no more," refuses,
I'll go no more:
I am afraid to think what I have done;
Look on't again I dare not.(2.2.64-66)
Angered at his fear, Lady Macbeth rebukes him as weak of purpose, taking from him the daggers. Then, there is knocking and Macbeth is shaken again. He looks at his hands, saying that all the water of Neptune's ocean cannot remove the blood, or guilt, from them. When Macbeth expresses this guilt, Lady Macbeth tells him that her hands are also red with the blood the King Duncan; she, too, is guilty of regicide as she has encouraged and aided Macbeth in this murder. However, she adds, she would be ashamed to have such a "white heart" as Macbeth; that is, to be so terrified at having committed the act. For, she mitigates any danger from the sleeping and the dead, whom she contends are but "as pictures."
It is ironic that in the beginning of their murderous path, Lady Macbeth is much more the aggressor and encourages their bloody deeds, seemingly more controlled and unconscionable. However, as the play progresses Lady Macbeth loses her mind in her torturous guilt for her bloody actions and murderous thoughts.