How about allowing people to choose who to kill? Maybe add an option to kill healthy young male adults who may serve as recruits? 
Let's say there is a region with 10 000 people. Out of that maybe 2500 are 'recruitable' population? The current option we have is just slaughtering anyone on sight I think but I do not believe they did that. Wars tend to leave orphans and widows but in BM they tend to leave nothing 
This seems an excellent idea to me. It would allow a winning realm to effectively destroy the war-waging ability of their enemy, forcing them to surrender, without actually destroying the whole realm.
