While they are exaggerating robber knights were a total thing where two nobles feuding would rob/kill each others peasants as part of it. Different times and places might have rules against it, the Holy Roman Empire very famously outlawed it but actual enforcement on the ground was minimal because there is nobody around to actually enforce those decrees. This isn't going into actual wartime conflicts at all in which case mass murder of non combatants was absolutely a common thing that was essentially sanctioned.