Macbeth is creating a very bad impression during Act 3, Scene 4. His guests cannot see the ghost he is talking to and therefore think he is insane. This would be extremely unsettling to all of them, since he has now become their king. Lady Macbeth, of course, knows that her husband is creating a scene that will remain in the memories of all the men who are present and that they will be discussing it for a long time to come. Macbeth's behavior presents a strong contrast to that of the noble King Duncan. It will undoubtedly weaken their loyalty to their new ruler. It might be said that this banquet scene is the beginning of the end for Macbeth. He reveals himself as unstable, and as time goes on the men he needs to help him maintain order in his kingdom will lose all their trust in him. Macduff is one of the first to turn against him. In time Macbeth will be losing most of his followers and will have no hope of defeating the English and Scottish forces invading his kingdom. Lady Macbeth herself is losing confidence in her husband, trying desperately to make excuses for him and finally asking everyone to leave, saying:
I pray you, speak not. He grows worse and worse.
Question enrages him. At once, good night.
Stand not upon the order of your going, But go at once.
The fact that Macbeth can see something that nobody else can see, including his own wife, makes him doubt his own sanity and feel all alone in the world. This isolation leads to his further paranoia and desperation.