If it's a minor npc you can absolutely say that when its broken it's dead as if it had suffered a mortal critical injury.
If a player is in a position to execute a coup de grace then that npc didn't die as above and I think the player should absolutely have to follow coup de grace rules if they want to kill it in cold blood. And this might be a great time to give the npc a name and make it into a more interesting character, just because it started as a "minion" doesn't mean it can't become a significant player in the campaign going forward.