How is stabbing a downed cavalry man any any more rude than him charging you from behind. And i would very much imagine a ranker would kill a downed cavalryman in real life and then rob his corpse.
Musicians well they annoy me so i kill them. I won't kill one that marching across the field on his own to see how far he will get just to see myself how far he will get but the ones in enemy positions are fair game as they are buffing their team.