After his servant leaves him alone, Macbeth sees, or thinks he sees, a dagger in the air in front of him. He says:
Is this a dagger which I see before me,
The handle toward my hand?
He draws his own dagger and lets the air-drawn dagger lead him off the stage. His last words before exiting are:
I go, and it is done. The bell invites me.
Hear it not, Duncan, for it is a knell
That summons thee to heaven or to hell.
Shakespeare evidently did not want to show the actual murdeer of Duncan onstage. Characteristically, he stresses words, poetry, iambic pentameter, rather than physical action. He gives Macbeth a long speech at the end of the scene in order to make his audience imagine what is going to happen in Duncan's bed chamber. Duncan is never seen with Macbeth when he is being murdered. Nor is he seen with Lady Macbeth when she goes to return the daggers to the scene of the crime and to smear the faces of the two grooms with blood. Nor is Duncan seen with Macduff when he discovers the King's body. The emphasis is always on words. Macbeth reenters the King's chamber after the discovery of the body, but here again the action occurs offstage. Violent action onstage usually looks faked. When Macduff has his death-duel with Macbeth at the end, most of the sword-fighting takes place offstage.
Macbeth is subject to hallucinations. The air-drawn dagger is just one of them. After he commits the murder he thinks he hears a voice crying "Sleep no more!" He thinks he sees Banquo's ghost at the banquet. Shakespeare seems to be trying to create some sympathy for Macbeth in order to make him seem more of a tragic hero than an out-and-out villain like Richard III. Macbeth is guilty, but there are extenuating circumstances--his hallucinations, his nagging wife, the Weird Sisters, and the air-drawn dagger seeming to show him that he has no choice but to go through with the murder of King Duncan.