Our fears are within Banquo, his royalty of nature. 'Tis safer to be that which we destroy, Than by destruction dwell in doubtful joy.
Put the thought of King Duncan out of your mind. Remember, you have to be bright and jovial in front of your guests tonight.
But we have scotched the snake, not killed it. We will eat in fear and sleep with nightmares, and O, full of scorpions in my mind, knowing that Banquo and his Fleace still live! They'll be done a deed of dreadful note tonight
What will happen then?
Be innocent of the knowledge. Good things of day begin to drop and drowse; While night's black agents to their preys do rouse
Lady Macbeth, just like Macbeth had done in the previous scene, voices her concern over Banquo. Her rhyming creates a tone of discontent, where Macbeth feels unsettled following the death of King Duncan even though that is what she desired.
The murdering of King Duncan has affected Macbeth emotionally. He metaphorically conveys that he lives in paranoia knowing that Banquo and Fleace are still alive. What Macbeth failed to realize when killed King Duncan, was that the violence would not stop there, now Macbeth must maintain his power as king through even more violent actions.
Macbeth foreshadows the murder of Banquo and Fleace through images of night and darkness. Although, Macbeth may be trying to protect Lady Macbeth from the truth by not directly stating what is going to happen.