Ambition

As Macbeth deliberates, he realizes that "vaulting ambition" is all that compels him to the heinous act of murdering Duncan and that his intent is nothing but personal gain.

" I have no spur
 To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other. (Act 1, Scene 7)

In Polanski's film and in history, being a king is seen as being linked to God. This was a temptation too strong for Macbeth to ignore, at least subconsciously.
Macbeth's subconscious ambition manifests itself in hallucinations, particularly the dagger.