Macbeth complains that dead men used to stay dead, but now they rise from the grave and annoy him.
When Macbeth and his guest gather for a banquet, he first stops to talk to the murderers to make sure Banquo and Fleance are dead. They tell him Banquo had his throat cut, but Fleance escaped. He is annoyed, but he goes back to his party. Unfortunately, he finds out the table’s full. The ghost of Banquo is sitting in his seat!
Macbeth panics. His guests and his wife do not see the ghost, so that don’t understand what he’s talking about. Lady Macbeth tells him he needs to cut it out, but he complains that dead men just aren’t staying dead anymore.
The time has been,
That, when the brains were out, the man would die,(95)
And there an end; but now they rise again… (Act III, Scene 3, p. 41)
As funny as this line is, it demonstrates that Macbeth is slowly losing it. He is becoming more and more out of touch with reality, and more and more paranoid. It is not guilt that forms the ghost, but fear. He sees Banquo as a threat, and is worried that he did not properly vanquish it by killing him.