In Shakespeare's Macbeth, in Act Two, scene two, after Macbeth has murdered Duncan (which he really did not want to do), the audience can tell that his actions have not come naturally to him. He is distracted, and has brought the bloody daggers back from Duncan's room.
Macbeth is extremely upset that when the grooms/guards were praying, that he could not say, "Amen." He says he really needed a blessing, but could not say "Amen." Of course, based upon Elizabethan standards, murdering a king—who was on the throne because God had ordained it—was a mortal sin. Macbeth has separated himself from God so it should be no surprise to him that he feels distanced from the Almighty.
One cried, “God bless us!” and “Amen” the other,
As they had seen me with these hangman's hands.
Listening their fear, I could not say “Amen,”
When they did say “God bless us!” (36-39)
Macbeth then turns his attention to the question of sleep—that a voice said that Macbeth had murdered sleep—the kind that refreshes one when he (or she) has worked a long day. The sleep that calms the mind; then the voice says that because Macbeth has murdered sleep, he will sleep no more.
Me thought I heard a voice cry “Sleep no more!
Macbeth doth Murder sleep”—the innocent sleep,
Sleep that knits up the ravell'd sleave of care,
The death of each day's life, sore labor's bath,
Balm of hurt minds, great nature's second course, (50)
Chief nourisher in life's feast—
What do you mean?
Still it cried, “Sleep no more!” to all the house;
“Glamis hath murdered sleep, and therefore Cawdor
Shall sleep no more. Macbeth shall sleep no more.” (55)
Macbeth is feeling extremely guilty for what he has done. Though he was one a noble man, he has—with one action—separated himself from the noble company of his peers and murdered his King and friend, separating himself also from heaven.
Lady Macbeth tells him to wash up and then he will feel better, but when she sees the bloody daggers he has brought back, she scolds him and demands that he return them back to where the dead king lies so it looks like his guards/servants are guilty of Duncan's murder. Macbeth firmly refuses, saying he will not go back. Again Lady Macbeth criticizes his lack of bravery, and his manhood, and returns the weapons herself.
Who was it that thus cried? Why, worthy
You do unbend your noble strength, to think
So brainsickly of things. Go, get some water
And wash this filthy witness from your hand. (60)
Why did you bring these daggers from the place?
They must lie there. Go carry them, and smear
The sleepy grooms with blood.
I'll go no more:
I am afraid to think what I have done; (65)
Look on't again I dare not.
Infirm of purpose!
Give me the daggers. The sleeping and the dead
Are but as pictures; ’tis the eye of childhood
That fears a painted devil. If he do bleed, (70)
I'll gild the faces of the grooms withal,
For it must seem their guilt.
When a knocking is heard, Macbeth is struggling with his guilt. Lady Macbeth has returned the daggers and makes fun of his fears. She is very calm about what they have done, saying that "a little water clears us of this deed..." (line 85)
Macbeth is not comforted: he knows the horror of his actions. His regret is evident when he wishes Duncan could be wakened with the knocking:
Wake Duncan with thy knocking! I would thou couldst!