Initially, Macbeth does not agonize over the moral implications of ordering the murder of Banquo and Fleance. He did agonize over Duncan’s murder. He orders Banquo’s murder out of fear. He believes that by killing Banquo, he destroys the prophecy that Banquo’s children will sit on the throne. However, at the banquet, Macbeth’s fear does turn to guilt and he sees or imagines Banquo’s ghost. This is significant because this is the first time Macbeth’s inner conflict is made public. This is when things really start to fall apart for Macbeth.
Macbeth becomes increasingly paranoid about everything. This is reflected externally as well as internally. He begins to think that Macduff’s absence at the banquet might imply that Macduff is plotting his own revenge.
It is also a significant break between Macbeth and Lady Macbeth. He sees the ghost and she does not. She tells his guests that he has a condition so they won't suspect anything. She simply thinks he is thinking too much.
They both have guilt and fear, but this is a breaking point. Remember that in Act III, Scene 2, Macbeth did not tell Lady Macbeth about the plan to kill Banquo and Fleance. “Such things” refers to Banquo’s ghost:
Can such things be,
And overcome us like a summer’s cloud,
Without our special wonder? You make me strange
Even to the disposition that I owe,
When now I think you can behold such sights,
And keep the natural ruby of your cheeks,
When mine is blanched with fear. (3. 4. 131-137)
These lines are also listed as 1409-1415 for the entire play. Macbeth wonders how she can witness such a thing as Banquo's ghost and not be affected by it. She doesn't see the ghost. She thinks he is consumed with fear, which is true.