Franz actively finishing off important characters

I am running a game and have been for a while. Ive devolped good relations with the members of my party. During one combat encounter Franz decides to actively kill an unconscious character. Why the absolute hell would that be a thing? I understand that its a mechanic in the game but if you go to any real world DM and ask them if they would do something like this the answer would be a resounding no unless they had a good reason to. Not just because its a mechanic in the game and they want to portray that world as a challenging and difficult world.