In this scene, Macbeth had recently assigned three murderers to kill Banquo. After Banquo's death, a murderer returned to Macbeth's banquet to alert Macbeth on the death of Banquo but that Fleance had unfortunately escaped. When the murderer left, Macbeth returns to his banquet, yet is greeted by Banquo's ghost who triggers a sudden feeling of fear and scare to Macbeth. Macbeth became so paranoid about the ghosts that Lady Macbeth was forced to cancel the banquet due to the behavior of Macbeth.
I believe that Macbeth is traumatized by the murder of Banquo. In Act three Scene three, the first murderer was curious to know who had joined the murderers to have a total of three. Macbeth must have been one of the murderers. Based on Macbeth's fear about the murder of Banquo in Act three Scene four, when Macbeth is talking to a murderer at the beginning of the scene, I believe he is talking to himself and is overly freaked out. Once Macbeth was nervous about the murder, he couldn't stop thinking about the murderer and developed constant stress. Also, considering the fact that Macbeth was the only one who could see Banquo's ghost, It determines that Macbeth was overly traumatized about murdering Banquo.
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