MACBETH
[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.

What this means is that Macbeth does not want to kill the King as he knows it’s wrong and he doesn’t want to see himself do it.