Rules of Warfare
One thing that “civilized” cultures like to do, is establish rules of warfare. Some are written, some unwritten, but it makes them feel better to know that they aren’t savages. So they act the part by saying “killing you neighbor over a border dispute is okay, but you won’t attack the medics trying to bandage them man who you just skewered.” While this might seem ridiculous we still do this to this day. Automatic weapons cutting your enemies in half – thumbs up, Napalm – thumbs down. Dropping huge bombs to obliterate entire buildings with all their occupants – thumbs up, simulating drowning – thumbs down. Well these nobles are no different then we are.
First Rule – Nobles are off limits – accidents happen – but they will not be targeted. There is an exception to this rule in Seige Warfare. Commoners are given a bounty for capturing enemy nobles (this changes from land to land, but is normally a few months to a years wages). The reason for this is threefold – Self Preservation, Greed, and Politics. Self Preservation is key, because most nobles want to live as long as possible so knowing that your enemy will try to spare your life if you try to spare his is a nice comfort. Greed is important because a duke pays little for the ransom of his brother if said brother is indeed corpselike. Politics is important for nearly the same reason as the greed. A duke is unlikely to attack you when you are holding his cousin as an “honored guest”.