Why do you think Macbeth imagines that he sees a dagger at the end of Act 2, Scene 1?
When Ross addresses Macbeth, telling him that he's just been given a Thaneship by the king, he describes the way that Macbeth behaved in the battle (he's talking to Macbeth himself). Macbeth, he says, was
It's a little, insignificant moment, but it's an important quote. Macbeth is going to become totally terrified of precisely that: of images which come out of his head.
The dagger which he sees, even as he sees it, he suspects might not actuallly be there. It might just be all in his head:
One of the things that this play is about is the way your mind can play tricks on you; Macbeth will later describe his mind as "full of scorpions". You can't control what you think. And we, as the audience, never quite know what has been created by the witches, and what is coming from Macbeth's h ead. Is it a witchy spell or a guilty spell for Macbeth? Impossible to say.
Yet you can well read it as an advanced version of guilt - Macbeth's mind is already starting to quiver at the thought of what he is going to do. And, of course, this happens again at the banquet - when Banquo pops up to horrify Macbeth (though invisible to everyone else). And Lady Macbeth draws out the comparison. Is it a real Banquo ghost, or just Macbeth's imagination? Who knows. But it's just like this dagger: