Banquo's ghost appears to Macbeth and nobody else at a banquet he and Lady Macbeth are hosting. Macbeth's reaction to the ghost spoils what was clearly meant as a gracious dinner for a group of honored guests, adding to the growing mood of horror and unease in the land.
Macbeth gets word during the dinner that Banquo has been murdered, which is what Macbeth secretly ordered. Banquo, he learns, was stabbed twenty times and left dead. Shortly after receiving the news, Macbeth imagines he sees the bloody corpse sitting on an empty stool at the banquet. Macbeth is openly and visibly upset, speaking to the guests of what he is seeing and wondering that they can sit there so calmly with rosy cheeks (as opposed to cheeks white with fear) despite a ghastly ghost in their midst. Lady Macbeth dismisses Macbeth's frightened words as one of the habitual "fits" Macbeth has had since childhood. Of course she is lying, as Macbeth's "fits" are a recent development. Because of Macbeth's outburst, the dinner party is ruined.
Guilt has overcome Macbeth, and the imagined ghost of his murdered friend haunts him. Although every murder is meant to benefit him in some way (by bringing him the crown, greater security, or peace of mind), ironically every murder simply makes his life full of more unhappiness, insecurity, and torment.
Macbeth has realized from the first that once he heads down this bloody path there's no turning back, and murder will lead to more murder, which is why he had to be goaded on initially by his wife to kill Duncan. Now he needs no urging. His decision is not to repent and pull back from this disastrous path but to double down. He tells his wife that his problem is that he is not yet hardened to murder, and that is why he reacts as he does to Banquo's death:
My strange and self-abuseIs the initiate fear that wants hard use.We are yet but young in deed.
Banquo's ghost appears in Act Three, Scene 4 to haunt Macbeth. After murdering King Duncan, Macbeth becomes worried that Banquo is suspicious of him and does not want Banquo's ancestors to become kings. Macbeth then sends assassins to kill Banquo and his son, Fleance. The assassins successfully murder Banquo, but Fleance escapes. Whenever Macbeth receives the news that Fleance is still alive, he begins to have doubts and fears. During a banquet, Banquo's ghost appears and is sitting in Macbeth's seat. Macbeth is unnerved and aggressively commands the ghost to leave him alone. Banquo's ghost is a manifestation of Macbeth's guilt and fear. Macbeth's hallucinations indicate that he is mentally unstable and the murders have irreparably damaged his mind and soul. Banquo is Macbeth's foil and is a morally upright, loyal individual throughout the play. Banquo's ghost reminds Macbeth of his sins, and Macbeth's reaction to the ghost depicts his moral depravity. At this point in the play, Macbeth is completely unhinged and is full of bloodlust, guilt, and anxiety.
The ghost is the manifestation of Macbeth's guilt and highlights Macbeth's moral downfall. He planned this murder on his own, unlike Duncan's murder. We feel disgust over this act; like Duncan, there is no real reason for this murder.