The banquet scene is both a high point for Macbeth as King of Scotland and the beginning of the end for him. The banquet is his first celebration as King, and he is joyous, thrilled with his new position, sitting on the throne suits him, he has managed to put aside the horrible images, the haunting images that plagued him right after Duncan's murder. At the banquet, Macbeth is determined to enjoy himself.
"Macb. Sweet remembrancer!
Now good digestion wait on appetite,
And health on both!" (Act III, Scene IV)
But, unfortunately, in the presence of those assembled, members of his court, his wife, surrounded by attendants,and Lords, Macbeth has a mental meltdown because he sees Banquo's ghost sitting on his chair, of course, no one else sees him. He becomes so unhinged by the sight of his former friend, now dead, thanks to his orders, that he can't focus on his party.
He can't sit at the table because the ghost of Banquo occupies his seat.
"Prithee, see there! behold! look! lo!
how say you?
Why, what care I? If thou canst nod, speak too.
If charnel-houses and our graves must send
Those that we bury back, our monuments
Shall be the maws of kites. [Ghost disappears." (Act III, Scene IV)
Once Macbeth sees the ghost, it disappears, but it comes back. And he addresses it again, causing all his guests to look in wonder at him. Lady Macbeth becomes concerned for her husband, he is the king, and he is making a fool of himself. So she orders everyone out, and breaks up the party claiming that Macbeth is having one of his spells.
"Lady M. Think of this, good peers,
But as a thing of custom: 'tis no other;
Only it spoils the pleasure of the time." (Act III, Scene IV)
This scene, in particular, underscores the fact that Macbeth still has a working conscience. He killed the king, inspired by the witches prophecy and with additional prodding from his wife, but he still had a sense of remorse after the act. By the time of the banquet, he has sufficiently recovered his control over his emotions to have a good time.
But his conscience is still working, so it intrudes on his party, because, Macbeth has, technically not by his own hand, murdered again. This time, his friend, who he perceived as a potential threat to his crown.
All the acts of murder that Macbeth hires killers to commit after he is crowned king are all motivated by fear or paranoia. He is desperate to protect his throne, one would assume that he thinks that he is going to live forever, the way that he tries to eliminate all potential successors to the throne of Scotland.