It is a minifestation of Macbeths deep buried guilt for killing his best friend. Of course, you could see Banquo's ghost as a 'real' occurence, but the fact that the rest of the retinue cannot see him suggests that this is a psychological phenomenon.
Macbeth behaves strangely at the banquet because he sees the ghost of Banquo, whom he had ordered to be murdered. This torments him as a manifestation of his guilt and fear, causing him to lose composure and expose his inner turmoil to his guests.
Macbeth says that he wishes Banquo was present at the dinner, yet Macbeth murdered Banquo. Macbeth proposed a toast to Banquo so that the Lords would not suspect Macbeth of any foul play. However, later on in this scene Macbeth betrayed himself. He thought he could see Banquos ghost, and he was acting strangely. The Lords were very confused and thought he was going mad and Lady Macbeth had to act for Macbeth and try and save their reputation.
Macbeth is so greedy that he kills duncan so he can be king
She tells him that he has to die so MacBeth can become king.
When LM read's Macbeth's letter, she immediately starts envisioning herself as queen. This influences her to start thinking of how to manipulate her husband Macbeth into killing King Duncan so the prophecy becomes true .
Macduff kills Macbeth. It says so in the script.
Yes. Lady Macbeth is the one that drugged the king's guards so they would be knocked out. She also plants the knives on the guards after Macbeth fails to do so.
Macbeth's character is so corrupted that he could not redeem himself even if he tried.
Macduff greets Macbeth's announcement that he has killed the grooms with the line "Wherefore did you so?" Macduff seems to think that was a peculiar thing to do, since he asks why Macbeth did it. Macbeth has a ready answer, but then in the next scene Macduff chooses not to attend Macbeth's coronation, so maybe he was not convinced by the answer.
Macbeth and lady Macbeth try to avoid suspicion by getting into their pajamas after committing the murder so people will think they were asleep all the time.
Macbeth kills the grooms so they cannot tell the others the truth.
There's no right answer. You have to decide for yourself who was responsible, which is what is so great about literature. There's either Lady Macbeth, the witches, or Macbeth himself. I believe Lady Macbeth was first to blame for her manipulation on Macbeth, then Macbeth because he chose to do the things he did under Lady Macbeth's word. The witches meerly told him what was going to happen in Macbeth's life.