What purpose does the scene between Macbeth and Banquo in Act 2 serve?
One unanswered question in Macbeth is “Why didn’t Macbeth kill Malcolm and Donalbain on the same night he killed Duncan?” Malcolm was the heir apparent. Duncan had specifically named him Prince of Cumberland and announced that he was his successor. And Donalbain would obviously be the next in line to the throne if Malcolm died. Macbeth's conversation with Banquo may enlighten us on this.
Just before Macbeth murders Duncan he encounters Banquo in Act 2, Scene 1, and they have a short discussion. The first bit is only intended to show that it is late at night, that the King is in bed asleep, and that he has “sent forth great largess to your offices.” This last will explain why everybody on the household staff gets drunk and why the Porter is so slow about answering the knocking at the gate. It also explains why Macduff, who is doing the knocking, was not accommodated inside the castle, although he is an important personage. Macbeth says:
Our will became the servant to defect,
Which else should free have wrought.
Then Banquo brings up the subject of the Weird Sisters, and 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.
At your kind’st leisure.
If you shall cleave to my consent, when ‘tis,
It shall make honor to you.
So I lose none
In seeking to augment it, but still keep
My bosom franchised and allegiance clear,
I shall be counseled.
Macbeth obviously would like help in the bloody coup he intends to commit. In these lines, he is subtly sounding Banquo out about the possibility of drawing him into the plot, but Banquo just as subtly lets him know that he will not take part in any treasonous affair, even though he has been promised by the Weird Sisters that his children will be kings and he can see that this could hardly come about without the elimination of Duncan and his sons.
So Macbeth immediately proceeds to murder Duncan. Time is of the essence. He will only have the King and his sons at his mercy overnight. Duncan may never return to Dunsinane again. Shakespeare establishes that Duncan has never been there before in the first lines of Act 1, Scene 6. With Banquo’s help it would have been easy to murder Malcolm and Donalbain in their beds, but, assuming Macbeth had intended to murder them by himself, he fails to carry out the triple murder because he loses his nerve.
When Macbeth sees Banquo's ghost sitting is his chair...???
It shows that Macbeth is guilty for betraying Banquo. Although Macbeth sent henchmen to murder Banquo and Fleance, the real threat to Macbeth's reign (Fleance) was not eliminated so he feels guilty that he murdered his best friend for nothing!! He ordered henchmen to murder Banquo becuase he was with Macbeth when the witches first told Macbeth his prophecy which also makes Banquo more likely to suspect Macbeth for murdering the king.