The actual murder of Duncan takes place between scenes, offstage. We hear Macbeth and Lady Macbeth planning to kill Duncan in Act I, and during Act II, we hear that the deed has been done.
In Act II, Scene I, Macbeth is awake late at night and roaming the castle grounds. He meets Banquo, who asks why he is not sleeping; Macbeth tells him he has been thinking about the witches. This is true, actually, because the witches' prediction that he'd be king is what motivates him to kill Duncan. At the end of the scene, he hears a bell. Macbeth says,
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. (II.i.19-21)
The bell means that Macbeth is now going to commit the murder, which is why it's Duncan's death knell.
When we next see Macbeth in Scene II, he is no longer calm and determined but panicked. He keeps claiming to hear strange noises and is very jumpy and on edge. Lady Macbeth
discovers that Macbeth still has the knife, the murder weapon. He was supposed to plant it on the guard, who they drugged so he'd be asleep during the crime. Macbeth cannot work up the nerve to go back and plant the knife, so Lady Macbeth has to do it herself. In this scene, we already see the toll the murder has taken on Macbeth. He cries,
Methought I heard a voice cry, “Sleep no more!
Macbeth does murder sleep”—the innocent sleep,
Sleep that knits up the raveled sleave of care,
The death of each day’s life, sore labor’s bath,
Balm of hurt minds, great nature’s second course,
Chief nourisher in life’s feast. (II.ii.35-40)
In addition to noticing his paranoia (hearing a voice), the audience can tell that Macbeth is feeling remorse and knows that he will never be the same. He will "sleep no more" because his conscience will never recover from this act. He will never experience the "Balm of hurt minds," sleep, since he will never feel at ease or at peace again. For Shakespeare's purposes, it is not important to see the murder itself, but it is crucial that we see the immediate impact of those events. Interestingly, Macbeth becomes rather hardened to this initial murder over time, while Lady Macbeth seems to be more psychologically damaged, as she clearly descends into madness. In Act II, Scene II, the opposite is depicted.