I'm not talking about the technical rules of legal moves when your king is checked. I'm talking about when there's checkmate and the victor and the loser are set in stone. Why can't I capture the king at that point? I can understand why you can't do so with a resignation because your pieces likely aren't near the king.
Whenever I play mountain blade, all the other nobles think I'm a bastard for immediately killing any noble who surrenders. But it allows me to get closer to strategic victory, and also the nobles have a habit of escaping before a ransom comes in