Don't quote me on this, but I believe it was because the knights had to maintain thier lavish lifestyle. To do this, they had to conquer or steal from the wealkings, the commoners. Obviously, the pesants didn't like being killed just for the knights to maintain their status, so they banded together to kill passing knights. The battle-ready pesants were called "godin." Because the pesants were willing to kill any knight, the knights had to kill any peasant they met to ensure their own safety. I hope that helps!
