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?
Macbeth didn't perform the deed directly, but he ordered the murderers to kill Banquo so he is just as guilty because he ordered the hit. Guilt isn't the only thing eating at Macbeth. Remember the prophecy the witches made about Banquo regarding his offspring? The weird sisters said Banquo would father a line of kings. The murderers did not kill Fleance--he escaped, and that weighs heavy on Macbeth's mind. Read Macbeth's lines just after the murderers tell him that Fleance escaped. Fleance is just a boy now, but he could grow up and one day avenge his father's murder, and take Macbeth's place on the throne. Macbeth's position is not secure and he's feeling a lot of anxiety over it.
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.