There are two major reasons why Macbeth is unnerved by his murder of King Duncan:
1. He feels guilt for having killed his kinsman, a virtuous king for whom he has held fond feelings, but now has betrayed. Previously, in his soliloquy of Act I, Scene 7, Macbeth wavers in his determination to commit the murder,
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host
Who should against his murderer shut the door,
Not bear the knife myself. Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office....
2. He has committed regicide. In the Elizabethan Age, there was great belief in the Chain of Being. So not only is regicide a crime; it is also a disruption of the order of the universe because kings are directly beneath deities (the divine right of kings): Macbeth fears "[T]he deep damnation of his taking-off." This is why Macbeth says in Act II, "I am afraid to think of what I have done." That he regrets having killed the king is further evinced in Act II,
To know my deed, 'twere best not know myself.
Wake Duncan with thy knocking! I would thou couldst! (2.2)
Further in Act II, Macbeth understands how terrible his act has been and regrets his regicide,
I had lived a blessed time; for from this instant
There's nothing serious in mortality.
All is but toys. Renown and grace is dead....(2.3)
In order to understand why Macbeth is nervous and tormented after killing Duncan, it is important to examine how he felt in the midst of undertaking the action. Macbeth is never quite settled in terms of having to commit murder. In the soliloquy he gives in the first scene of Act II, Shakespeare employs imagery to show the level of conflict that exists in Macbeth.
A dagger of the mind, a false creation,
Proceeding from the heat oppressed brain?
I see thee yet, in form as palpable
As this which now I draw.
Thou marshall'st me the way that I was going,(50)
And such an instrument I was to use.
Macbeth further indicates that "Mine eyes are made the fools o’ the other senses." It becomes clear that Macbeth does not really want to do this. He feels that he is losing control of who he is and what he wishes to do. His nervousness and sense of torment about what he has to do is rooted in the idea that he really does not want to do it in the first place. This is in stark contrast to how Macbeth will appropriate the idea of murder and slaughter as the play develops.
When Macbeth returns after "having done the deed," his responses to his wife are terse and lacking depth. This helps to convey a sense of emotional ambivalence that exists within him. His nervousness is a reflection of how he is unclear as to why he has done what he has done as well as the wide implications of what he has undertaken. When Lady Macbeth tries to dismiss her husband's nerves in her line of "Consider it not so deeply," it does little to assuage the condition of nerves that her husband possesses. When Macbeth is convinced that he heard someone say, “Sleep no more! Macbeth hath murdered sleep," upon killing Duncan, it is a reflection of the guilt he feels and the emotional torment. His nerves are shot because he is unable to process what he did. Shakespeare shows that human regret and guilt often accompanies human transgression. The same factors that compel individuals to embrace such activities are not present in assuaging the guilt feels over doing what one felt the need to do.
First you must know that Macbeth was not a murder and would not have done the deed in the first place, but the prophecy and Lady Macbeth (who called him a coward) convinced him to do so. He was very guilty after killing the king and knowing he had done it put him on edge. There were suspicions in the castle so he had to be careful and he was afraid people would find out what he had done. He had committed a horrible crime and he regretted it. That made him hallucinate and have horrible dreams.