MacBeth clearly becomes frighteningly unhinged during the banquet scene. He is hallucinating, which indicates that the guilt he feels has taken control of his mind. Banquo's ghost gives him no peace and keep popping up at the dinner table at the most inopportune moments. Lady Macbeth tries to cover for him, but eventually calls it a night and tells the guests that they must leave. She is afraid that Macbeth will ruin everything by revealing the deed in his rantings with the bloody ghost. Macbeth is bordering on madness and deeply fearful that he will be found out and lose the throne he has gained through murder and deceit. Remember that he feels guilty - he knows he has lowered himself into the belly of the beast at this point.
Macbeth discovers Banquo's ghost and talks with him. Of course, the guests cannot see him. Lady Macbeth assures the guests that he has had these spells since his childhood. She attempts to control him, and host his guests but is unsuccessful. Finally, she asks the guests to leave so that he can rest.