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.
The terrible desire is to kill anyone in his way so that he can become king. That would include killing the king and his son who has just been named the Prince of Cumberland.