In act 2, scene 1, Macbeth sees a vision of an airborne dagger floating in front of him, pointing the way to Duncan's bedchamber. He apostrophizes the ghostly apparition, saying,
Thou marshall'st me the way that I was going;
And such an instrument I was to use.
By Macbeth's own account here, the illusion of the dagger has not influenced him much, if at all. He was already going to kill Duncan, an idea he conceived in act 1, scene 4, after Malcolm was made prince of Cumberland.
There are three principal illusions in the play. The first is the airborne dagger, the second, the ghost of Banquo, and the third is the selection of apparitions to which Macbeth is treated in the witches' cave in act 4, scene 1. It could be argued that Macbeth kills King Duncan in response to the first illusion, visits the witches and thinks of killing Macduff in response to the second, and actually attacks Macduff's castle in response to the third. The apparitions the witches show him also give him confidence in his "charmed life," invulnerable to moral swords.
In fact, all the illusions in the play are like the dagger, in that they marshal Macbeth the way that he was going. Macbeth is an eager convert to villainy, tyranny, and sorcery, and the illusions he sees are precisely the ones he wants to see, giving him further excuses to pursue the course on which he had already decided.