Banquo, like Macbeth, is awake long after Duncan and the other guests have gone to sleep. Banquo is feeling depressed. He tells his son:
A heavy summons lies like lead upon me,
And yet I would not sleep: merciful powers,
Restrain in me the cursed thoughts that nature
Gives way to in repose!
He is afraid to go to sleep because he can repress his "cursed thoughts" while he is awake but not if he falls asleep. He has been told by the Weird Sisters that he will become sire of a whole line of kings. This has made a strong impression on him despite his loyalty to Duncan. Banquo would like very much to have his son and grandsons become kings.
Duncan is an old man. He is sound asleep. It would be easy to kill him. Yet how would that expedite the process of Banquo's heirs becoming kings? Macbeth has a similar problem. If he kills Duncan, then Malcolm would become king, and Donalbain would be next in line.
Then the two men meet in the dark, silent palace and have a conversation full of hidden implications. Banquo brings up the subject on both their minds:
I dreamt last night of the three weird sisters:
To you they have show'd some truth.
This proves that his "cursed thoughts" are related to the witches. He knows he will dream of them again, and they will repeat their prediction that he will be the progenitor of a whole line of Scottish kings. And this will lead him to the murderous thoughts he dreads.
Macbeth has gone beyond the stage of treasonous thoughts and cursed dreams. He is planning to murder Duncan in his bed that very night. He thinks he might persuade Banquo to cooperate with him. After all, they both have the same reason for wantinig Duncan dead. Macbeth says:
I think not of them:
Yet, when we can entreat an hour to serve,
We would spend it in some words upon that business,
If you would grant the time.
This is wonderful dialogue. Macbeth is asking Banquo to conspire with him without putting the proposition into plain words. Since Banquo answers that he would be glad to talk to him--(and what better time would there be than right now when they are alone and everybody is asleep?)--Macbeth goes a step further:
If you shall cleave to my consent, when 'tis,
It shall make honour for you.
But Banquo makes it clear he will not consider what he knows Macbeth has in mind. Macbeth would dearly love to have Banquo on his side because he would certainly like to kill Duncan and both Duncan's sons while he has them under his roof for this one night. It would seem that if Banquo had agreed to talk about a bloody coup, the two men would have killed Duncan, Malcolm, and Donalbain that night. Then Macbeth could have claimed the throne and Banquo could have supported him.
Macbeth realizes he has to act alone. Whether or not he intends to kill Duncan's sons is never made clear. He could not have anticipated that they would both flee for their lives and he could blame their father's murder on them. He should have killed them--but he didn't have the nerve to do it alone. He makes excuses to his wife in Act 2, Scene 2. He says the grooms woke up while he was in the King's chamber. Then he heard a voice shouting "Sleep no more!" Then there was that persistent knocking at the gate. Shakespeare seems to be explaining why Macbeth didn't complete the triple murders.