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.
Blind ambition is a state that fails to take into account the obvious obstacles and dangers associated with it. In this play the moment the witches addressed Macbeth 'as king hereafter. ' (Act 1 Sc. 111,50.)he wanted to be king.He asks the witches to tell him more and wants to know where they got their information from.He is even questioning Banquo's thoughts. He asks him if he hopes his children shall be kings. He rationalises with himself the validity of his growing ambition when he in his aside: 'Two truths are told ,as happy prologues to the swelling act". (Act 1, Sc4,127.) It seems that he has already made up his mind to kill the King.
Over this time he admits to reconsidering his ambition. He admits to that the idea of murder is yet only fantastical and asks himself that if 'chance had brought him so far perhaps chance will have him crowned. ( Line 143) He also ponders his duty of loyalty to King Duncan and admits that it is protecting him he should be.
However , his own fierce ambition together with the encouragement of his wife overrules everything . He imagines a dagger in front of him as the omen to go ahead and do the deed . He decides
I go and it is done ;the bell invites me.
Hear it not Duncan, for it is a knell
That summons thee to heaven or to hell. (Act 2, Sc1,63-65)