There's effectively no difference right now, except that lethal attacks do extra damage. Down the line, this will change. At a very minimum, a lot of killing will result in a darker ending. The next addition, which is pretty likely, would be each ally having an attitude to killing. Some will be against it (Ray Case, Emilia DeSoto), others neutral (Arcadi Konstantin, Edgar Kowalski, Caia Winters), while others (Elena Furey) have a preference to kill. Forcing an ally out of their preferences will cause damage to your relationship. This would likely be most severe if you force them to do it, less severe if they witness someone else doing it, and less severe if you do it when they aren't around. Building on this, it's possible that certain characters may switch combat modes between turns, forcing you to keep an eye on them, and have to pass a check to switch them back.
