Banquo has a few reasons ready to serve up to the murderers he's hired to Kill Banquo and Fleance. His first one is that Banquo is the reason for their poverty and miseries, "Know That it was he in the times past which held you So under fortune..."Macbeth rambles on about how they are so poor and "underfortune" with no prospects because of that damn Banquo. The murderers are playing their part. They really don't know Banquo from a hole in the ground and could care less what happens to him; they just want their money. Still, they have to let Macbeth blather on about Banquo being the reason they look like pig slop and smell like dung. I's pretty obvious that this whole exercise in "blame Banquo" is to abate Macbeth's own fragile conscience. Although killing people is getting easier for him, Macbeth still needs to manufacture a just reason. Macbeth's attempts to put his killing rampage in a righteous light is an example of his crumbling psyche.