In Act I.ii or Act II.ii, or perhaps Act II.i ?
In Act I, scene II Macbeth does not appear. The Bleeding Captain describes his battle in a monologue. He does not entertain thoughts of murdering Duncan, as he has not encountered the witches yet
For brave Macbeth--well he deserves that name--
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valour's minion carved out his passage
Till he faced the slave;
Which ne'er shook hands, nor bade farewell to him,
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements.
In Act II, scene II Macbeth has already killed Duncan.
In Act II, scene I Macbeth sees the imaginary dagger:
Is this a dagger which I see before me,
The handle toward my hand? Come, let me clutch thee.
I have thee not, and yet I see thee still.
Art thou not, fatal vision, sensible
To feeling as to sight? or art thou but
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain?
Here, He is battling between thought (imaginary dagger) and action (real dagger), the real act of murder and the thought of murder. He is battling between the id (actual dagger) and the superego (imaginary dagger). He is battling between the supernatural (imaginary dagger) and the unnatural (real dagger). Later, after he sees Banquo's ghost, Lady Macbeth will say the ghost is:
So, the imaginary dagger is one of fear, while the real dagger is one of--in her eyes--courage (i.e., "screw your courage to the sticking place")