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.