Although we never see the actual murder, Macbeth kills Duncan at the end of act II, scene I, and Duncan is dead by the next scene. The murder is preceded by Macbeth's indecision over whether to go forward with Lady Macbeth's plan to assume Duncan's throne.
In fact, in act I, scene VII, set in Macbeth's castle where Duncan is a guest, Macbeth is still not committed to the murder. He questions his wife about the plan, asking her what will happen to them if they fail in their plot. Her reply to him is intended to reinforce his courage,
But screw your courage to the sticking-place,
And we'll not fail.
Although the scene is not staged for the audience, Macbeth kills Duncan later that night. We know that Duncan is about to die because in the final moments of act II, scene I, Macbeth hears a bell ring and says to himself,
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.
Moreover, the very next mention of Duncan makes it clear that he is already dead and that, at this point, Macbeth regrets what he has done. Macbeth hears loud knocking and thinks to himself that if Duncan were alive, the knocking would wake him. Macbeth thinks,
Wake Duncan with thy knocking! I would thou couldst!
Macbeth wishes that he could undo what he has done. It is too late. Duncan is dead and from this point on, Macbeth descends into a cold spiral of remorseless ambition. The Macbeth that emerges after Duncan’s murder is a far cry from the earlier man who was grateful to Duncan for all that he had bestowed on Macbeth. The earlier Macbeth even tells Duncan that he owes him his service and loyalty and his effort to keep Duncan safe.