Answer: Macbeth is restrained from the murder of Duncan by the power of a sensitive conscience, working through imaginary terrors. Notwithstanding the assuring prophecy of the Weird Sisters, he is still haunted by the dreadful fear of the unknown, possible consequence. Immediately after the murder, conscience is still more active, and he cannot bring himself to face the horrors which imagination conjures up, — he cannot brave that “voice” again, — he dare not look on the murdered Duncan! It is the natural terror of a man “but young in deed,” — “the initiate fear that wants hard use.” His prompt murder of the grooms in the very next scene, though seemingly, is not really at variance with this shrinking which we have just noted.
By utmost effort his wife has, in the interval, succeeded in rousing him to a realization of the immediate danger of detection in which they stand. Impressed with this idea, he comes forth to meet the nobles, and to play such a part upon the discovery of the murdered King, as shall entirely disarm suspicion. His whole conduct is governed by this desire, and is just what we should expect from a man whose face is “as a book where men may read things strange.” His very language is strained and unnatural, appropriate only in the mouth of a conscious murderer dissembling guilt. He talks to avoid his own thoughts, and to mislead others.
Exhibition of great grief for the death of the king and hatred for the perpetrators of the horrible deed seems to him the proper course, and in no way can this pious indignation be so effectually shown as in slaying the supposed culprits. It is possible, too, that he feared the grooms, who had been in the chamber, certainly roused, and may have seen more than he supposed.