Throughout the murder scene of Shakespeare's Macbeth reveals his weakness but also his strong conscience. Because of Lady Macbeth's persuasiveness, he finds the courage to kill Duncan, but he now shows his fear, sorrow, and regret for the assassination of Duncan. When Macbeth arrives in the scene after having killed Duncan, he is shaken when he thinks he hears someone because he fears being caught. When he is leaving the bloody murder scene, he starts, hearing what Lady Macbeth assures him is only "the owl scream[ing] and the crickets cry[ing]" (II.ii. 20), but he is afraid that he hears Donalbain or Malcolm, "who lie i[n] th[e] second chamber" (II.ii. 26). Lady Macbeth tells him that he is foolish to be afraid, but he insists that he heard "[one of Duncan's sons] laugh in [hi]s sleep, and one [cry]/ 'Murder!'/ [and] that they did wake each other" (II.ii. 30-32). Lady Macbeth, who persuaded him to commit the murder, is confident that Macbeth would not get caught, and Macbeth is soon past his fears.
Though Lady Macbeth downplays Macbeth's fears of being caught, her reassurance does not convince his guilty conscience. Macbeth, knowing that he committed a horrible crime and sin in regicide, feels so guilty that he "could not say 'Amen'/ when [Donalbain and Malcolm] did say 'God bless us'" (II.ii. 39-40). He knows that his crime was so awful that God would never forgive him. When "[he] thought he heard a voice cry 'sleep no more!/ Macbeth does murder sleep'" (II.ii. 47-48), he says that not only will he never be forgiven by God, but that he will never forgive himself for committing regicide.
In the murder scene, Macbeth finally realizes the gravity of his crime. This is when he realizes that the murder was morally wrong, not only illegal, and not simply the only path to power. Macbeth now knows that he will regret his deeds for the rest of his life, as "all great Neptune's ocean [will not] wash this blood/ Clean from [his] hand" (II.ii. 78-79).