Macbeth plans to murder Duncan and then pretend to be sound asleep when the body is discovered in the morning. However, because of the incessant knocking at the gate he is forced to put in a personal appearance to find out why nobody is responding. The drunken porter opens the gate just before Macbeth arrives and explains to Macduff that all the servants and guards were drunk. Why do Macbeth and his wife tolerate such behavior, especially when hosting the King, his two sons, and many notables?
When Banquo encounters Macbeth at the beginning of Act 2, he provides an explanation for questions that will arise later.
What, sir, not yet at rest? The King's abed.
He hath been in unusual pleasure, and
Sent forth great largess to your offices.
This "great largess" included enough liquor to get the entire household staff dead drunk, which will explain the prolonged knocking next morning. Shakespeare wanted Macbeth to be present when Macduff discovers the King's body. The fact that the King himself supplied the liquor will prevent Macbeth from punishing his servants for getting drunk.
Macbeth next replies to Banquo:
Our will became the servant to defect,
Which else should free have wrought.
This is intended to explain why Macduff was not accommodated inside the castle but had to sleep outside in a hovel, and why he will be doing all the knocking in the early morning. It does not explain why Duncan should have appointed Macduff to wake him, but perhaps neither Duncan nor Macduff knew that Macduff would have to spend the night outside the castle.
Then Banquo brings up the subject of the Weird Sisters. Macbeth would obviously like to draw Banquo into his assassination plot. It would seem that Macbeth cannot accomplish his purpose merely by murdering Duncan, because Malcolm is next in line for the throne as Prince of Cumberland. After Malcolm would come Donalbain. Macbeth has the unusual opportunity of having all three under his roof and would probably like to murder them all that very night, but he can't handle it alone. This seems to be why he is sounding Banquo out.
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.
At your kind'st leisure.
If you shall cleave to my consent, when 'tis,
It shall make honor for you.
So I lose none
In seeking to augment it, but still keep
My bosom franchised and allegiance clear,
I shall be counseled.
Good repose the while.
There will be no future discussion. Macbeth already realizes that Banquo will never join him in an assassination plot. So he immediately goes ahead with the murder of Duncan. However, he cannot murder Malcolm and Donalbain too. He may have planned to do that, but at least two things held him back. For one, he was so sickened by murdering the King, whom he loved and who had treated him so kindly, that he retreated to his chambers and let his wife take the daggers back to smear the faces of the grooms with blood. For another, he imagined that he heard a loud voice shouting, "Sleep no more! Macbeth doth murder sleep!" He was afraid the voice would wake people up and he would be found covered with blood and holding the two murder weapons. He could not have foreseen that Malcolm and Donalbain would flee for their lives, giving him the opportunity to blame the King's murder on them.