When the audience is introduced to Macbeth in the second scene of Act I, he is the hero of the fight against the rebel Macdonwald, who he is described as having personally dispatched in battle. He is described by Duncan as a "worthy cousin." The pivotal scene of the act is when Macbeth and Banquo encounter the witches, and while taken somewhat aback, he is clearly intrigued when they hail him as thane of Cawdor, Glamis, and the future king of Scotland. When he discovers in the same scene that he has, in fact, been given the title of Thane of Cawdor, he begins to think about the implications:
This supernatural soliciting
Cannot be ill, cannot be good. If ill,
Why hath it given me earnest of success,
Commencing in a truth? I am Thane of Cawdor. ... If chance will have me king, why, chance(155)
may crown me/Without my stir.
One scene later, the audience learns that Macbeth is contemplating something wicked, though he hopes to keep it a secret. He describes Malcolm, the heir to the throne, as a step he will have to overcome.
The Prince of Cumberland! That is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires...
Lady Macbeth famously gives him the push he needs to commit the murder of Duncan, and he is clearly torn about committing the deed in the seventh scene, when his wife questions his manhood. But by the end of the scene, he says he has committed "every part of my body to this terrible event." He has gone from an ostensibly loyal and highly regarded vassal of King Duncan to a man who is willing to commit murder to achieve his ambitions.