Macbeth's strongest reaction to Duncan's murder does not come over him until Macduff discovers the King's dead body and raises a general alarm. Macbeth didn't want to be present when this happened, but the knocking at the gate forced him to stop playing possum and come down to see why nobody was opening the gate. Macbeth expresses his feelings in the following lines in Act 2, Scene 3:
Had I but died an hour before this chance,
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.
The wine of life is drawn, and the mere lees
Is left this vault to brag of.
He wishes he were dead. He is overcome with guilt and remorse. He realizes that he has made a terrible mistake and that he is going to have to live with his guilt and shame for the rest of his life. The metaphor about the wine of life suggests that all the wine has been drained from the barrel and only the bitter residue is left at the bottom. He can't see how anything he can do for the rest of his life can bring him any pleasure or satisfaction.
I completely agree with the above posts and would add that Macbeth actually has two reactions to Duncan's murder: his genuine reaction, which the above posts discussed, and also his phony reaction to the king's death, staged for the benefit of the public. When MacDuff announces the king has been murdered, Macbeth feigns ignorance to the deed and asks confusedly, "What is't you say? the life?" (II.iii.74). After he pretends to be shocked by the recent turn of events, Macbeth takes on the role of wounded host, deeply bitter that such a travesty would occur in his household; he claims to have killed the servants, the very ones that he intended to frame with the bloody daggers, in a fit of rage:
"Who can be wise, amazed, temperate and furious,(120)
Loyal and neutral, in a moment? No man:
The expedition of my violent love
Outrun the pauser reason" (II.iii.120-123).
Macbeth constructs an elaborate show of emotion for his guests, hoping that his elaborate ruse will avert their suspicion.
When Macbeth kills Duncan, he gets very anxious. He imagines that he hears the guards talking in their sleep.
There's one did laugh in's sleep, and one cried,(30)
That they did wake each other: I stood and heard them:
But they did say their prayers and address'd them
Again to sleep. (Act 2, Scene II)
He gets even more upset when he thinks he hears them say “amen” and feels the words get caught in his throat. He wonders why he cannot say it back. It makes him feel that what he has done must have been sinful. He gets so confused that he forgets to leave the daggers to frame them.
It does not take long for Macbeth to get over his guilt though. He is soon killing people left and right to protect his throne. It demonstrates the transience of guilt in the play. For Macbeth and Lady Macbeth, it comes and goes.
Immediately after murdering Duncan, Macbeth experiences a combination of remorse and panic. He says that he has heard a voice saying "Sleep no more! Macbeth doth murder sleep." He is so out of sorts that he has left the bloody dagger he used to kill the king at the scene of the murder, and he refuses to return to pick up the murder weapons. His wife accuses him of being "infirm of purpose," and does it herself. Macbeth's reaction, particularly the spectral visions and voices that he experiences, foreshadow his response to murdering Banquo, and his wife's sleepwalking visions immediately before her suicide.
After murdering Duncan,Macbeth becomes aware of the sin he has committed and the goodness and peace he has lost for ever.He cannot say 'Amen' when he hears a prayer.He feels that Duncan has escaped all cares and worries of life which he is left to bear.He has to carry his cross.He contrasts Duncan's serene face with his own guilt laced visage silently and says,"There lies Duncan/After life's fitful fever he sleeps well."He is jealous that Duncan has escaped the fever and frets of life to death's dateless night where he will lie in eternal sleep.