Much of Macbeth's soliloquy at the opening of Scene 7 in Act 1 describes his blind ambition. He tells himself that he knows he knows he cannot escape the consequences of assassinating Duncan.
But in these cases
We still have judgment here, that we but teach
Bloody instructions, which, being taught, return
To plague the inventor.
He tells himself that he is committing a villainous action because he is Duncan's kinsman, his subject, and now his host
Who should against his murderer shut the door,
Not bear the knife myself.
He concludes the soliloquy by acknowledging that he has no reason for killing Duncan other than blind ambition. He uses an unusual metaphor in which he compares himself to a man jumping onto the back of a horse but falling off on the other side rather than riding the horse correctly. In the metaphor it is his intentthat he compares to the horse. He intends to commit the murder even though he has no way to justify or rationalize the deed.
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on th' other--
Lady Macbeth enters at this point, apparently interrupting him from finishing the line with the word "side." Her entrance at this point carries a strong implication that it is his wife who is driving him on to do the deed which he knows is an outrage and a bad mistake.