The scene of Duncan's murder relates to the theme of betrayal, especially in the way that Macbeth, who should be Duncan's host and loyal lord, is the one who commits the murder. Macbeth certainly has misgivings about committing the crime all the way up to the moment of the murder. He fears retribution for his act, which to him seems a violation of Duncan's trust and good faith. In Act 1, Scene 7, Macbeth frets over his plan and acknowledges the implied betrayal of his actions:
[Duncan's] here in double trust:
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,(15)
Not bear the knife myself. (I.vii.13-16)
Duncan's murder is steeped in betrayal, for the reasons listed by Macbeth himself in the above quote; he betrays the king both in terms of being his subject as well as his host.
After Macbeth murders Duncan in his sleep, he cannot bring himself to return to the scene of his betrayal:
I'll go no more:
I am afraid to think what I have done;(65)
Look on't again I dare not (II.ii.64-66)
Macbeth's repulsion at the crime he has committed convicts and terrifies him. He laments that "Neptune's ocean" might not have enough water to cleanse the blood from his hands; his guilt at the betrayal he has committed against the king threatens to overwhelm him.