After the murderers kill Banquo, why did Macbeth go insane seeing the images of him if he didn't kill Banquo and have to live with the guilt?

Guilt is a horrible thing.  It is not just the murder of Banquo that is preying on Macbeth's mind.  He has Duncan on his conscience as well, and he is already cooking up the deaths of the MacDuff family and all their servants and houseguests.

Of course, any time you kill someone and he shows up all bloody at your dinner party, that would be enough to go a little over the edge.  Macbeth did not stab Banquo and attack his son, Fleance, but he hired it done.  Banquo's blood is on Macbeth's hands regardless.  Remember, too, that with Duncan's murder, Macbeth's guilty conscience is already manifesting itself in the form of many sleepless nights.