they are still nobles. If you can kill one, you can kill others. So yes, that action will have consequences.
in wars they try to keep the high officers / nobles alive and ransom them. It works both ways. It makes the soldiers disposable tools, and the big shots the exception of the rule (more likely to survive).
they are the ones calling the shots, so yeah, they want a system where they are less likely to die .. even in defeat lol. Just a natural thingy.
executing should be limited to a few nobles which you have a beef with, not the entire kingdom, unless you want to become hated by the rest of the world ... which is fine, just take the hit and do your thingy.
its early EA, so yeah, its not even close to final. This particular issue was even recently discussed by devs.