Macbeth Act 1 Scene 4:
[Aside] 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:
The eye wink at the hand; yet let that be,
Which the eye fears, when it is done, to see.
Having just won praise and new honours from the king for his fight against the invading forces of Norway Macbeth is told the news that The Prince of Cumberland title (next in line to the throne) is to be awarded to Malcolm, son of Duncan. At this point comes the stirrings of evil within Macbeth. previously he has been thrilled to find that chance has made him Thane of Cawdor, if that is the case then chance may well make him king as well if he does nothing. But now with this news his chances are slimmed down, it presents him with a major obsticle.
Perhaps at this point Macbeth secretly begins to contemplate an act that will bring the witches prophesy true, he certainly begins to look toward hiding his true desires and covering his thoughts.
Lady Macbeth also shows the intent of her greed in the next scene: Act 1 Scene 5:
Glamis thou art, and Cawdor; and shalt be
What thou art promised: yet do I fear thy nature;
It is too full o' the milk of human kindness
To catch the nearest way: thou wouldst be great;
Art not without ambition, but without
The illness should attend it...
The illness that she refers to is the illness of ambition, greed and the desire to do everything necessary to make things happen.