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 no light see my black and deep desires.
Once Macbeth found out that Duncan wanted Malcolm as his successor, that made Macbeth upset. Since two out of three of his earlier predictions made by the witches have been done, he believes it's a possibility to be king, but once Malcolm came into the picture, Macbeth knew that he had to kill him or give up.
