In "Macbeth," why is Macbeth so upset by Banquo at the feast?

Macbeth is upset because what he sees isn't a body--it's a ghost.  He has seen and created many bodies, but this is no body.  He says at one point:

...The time has been

That, when the brains were out, the man would die,

And there an end.  But now they rise again.... (Act 3.4.78-80)

Banquo is dead, but apparently doesn't want to stay that way.  This frightens Macbeth so that he once again behaves in the way that his wife repeatedly tells him not to:  like he is terribly guilty of something. 

In the end, she chides him for throwing another fit, and dismisses his antics, telling him that he looks on nothing but an empty stool.