In Act 2, scene 2, Macbeth has just murdered Duncan. He is disturbed and agitated by what he has done, pumped full of adrenalin and in a state of heightened emotion and awareness. His conscience is so stricken by the deed that he can't say the word "Amen." The extraordinary guilt his conscience experiences over killing a good king pushes his imagination into overdrive: he thinks he hears a voice crying out "Sleep no more! Macbeth does murder sleep." He also imagines he hears knocking and wonders why every noise alarms him and sets his nerves on edge: "How is 't with me when every noise appals me?" He then envisions that no amount of water can wash the blood of this murder from his hands, picturing instead a situation in which the blood will turn the entire green sea red:
Will all great Neptune’s ocean wash this bloodClean from my hand? No, this my hand will ratherThe multitudinous seas incarnadine,Making the green one red.
Lady Macbeth tries to pooh-pooh his fears and bring him down into a more rational state of mind, but Macbeth can't hide from himself that he has committed a morally reprehensible act. He has murdered a good king who has just honored him and to whom he owes his loyalty. Moreover, he has done the deed while he is supposed to be offering Duncan hospitality and protection in his castle. All of this violates the moral norms Macbeth has long internalized. He knows and vividly expresses that he will never rest easy again.
In Act 2 scene 2, Macbeth has just killed Duncan and is staggering back to Lady Macbeth covered in his blood with the bloody daggers in his hands. Lady Macbeth scolds him for not leaving the murder weapon planted on the guards as he was supposed to. The fact that he cannot return the weapons himself after realizing his mistake is one thing that illustrates that he is feeling guilty. He cannot go back to face the dead body. In addition, he keeps muttering about all the blood on his hands, coming from the king, on the daggers, this also shows the blood as a symbol of his guilt.
As for his imagination, Macbeth hears noises when he first comes in imagining people in the castle that could be witness to his crime. He furthermore goes on to talk about how when he was committing the crime, he thought he heard someone say "Sleep no more . . . Macbeth has murdered sleep" (2.2.42-44). Hearing voices also is an indication of his conscience kicking in and making him feel guilt.