In Shakespeare's Macbeth, when does Macbeth kill Duncan?
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 knellThat summons thee to heaven or to hell. (II.i.19-21)
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 Shakespeare's Macbeth, as is typical in Shakespeare, violence takes place offstage. The audience does not see Macbeth kill Duncan any more than we see Macduff kill Macbeth later in the play. At the close of Act II, Scene 1 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.
Then he exits. He returns to stage in scene 2 after only a brief absence and tells Lady Macbeth, "I have done the deed." The results of the assassination are revealed to the audience for the remainder of the play, but the act itself is not seen.
Just so you know, I edited the question in order to reflect the accepted convention of referring to details within a work of art in present tense rather than past tense. Thus, "does" works better in the question than "did." When you analyze literature, think of it as if it's happening whenever it's being read, or in this case, maybe, performed.
I am not completely clear on what you mean by this question. Do you just mean at what point in the play did Macbeth kill Duncan? I can not think of any other way to understand this question.
Anyway, Macbeth kills Duncan in Act II. You could say he does it in Scene 1, or you can say he does it in Scene 2, or you can say he does it in between. At any rate, we do not see him do it.
At the end of Scene 1, he has just said that it is time to go and kill Duncan (because the bell rang). At the beginning of Scene 2, he comes in and tells his wife he has killed Duncan. So somewhere in between those two things, he killed Duncan.