In Act 1, Scene 4, Macbeth reacts to the announcement that Malcolm is the Prince of Cumberland (the title of the heir to the Scottish throne) by revealing his ambition to gain the crown himself:
The Prince of Cumberland! That is a step(55)
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:
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see.
His "black and deep desires," the audience soon discovers, will lead him to do horrific things. Many observers have argued that Lady Macbeth alone spurred him to commit murder and usurp the throne, but it is clear in this scene that he already has these ambitions. He is already thinking about what he has heard in the previous scene from the witches: that he is to be King of Scotland, and the idea has clearly struck a nerve.