Next time I run a game, I'm going to read the following statement:

Good evening. I am your game master and I reserve the right to kill your character, or not, as I see fit. Your opponents will try their best to kill, maim or capture you, as their preferences allow. They are not your friends and I will run them as deadly as I am able. Rest assured, I will not kill your character in a capricious or ill-conceived manner. I will attempt to have your consent on this but not always. In the event that an opponent scores enough damage to actually kill your character, I will choose whether to keep him alive in order to screw him later or let the death stand. The odds, however, favor neither choice, but I will admit an opportunity to mess with you in the future is more appealing. In fact, the rule will be that the each time I chose not to kill the character despite the roll, the fates will be unkind to them sometime in the future, in order to balance the scales. Their cheating death will translate to a complete and utter failure that I can bestow on any action your character takes. He wants to impress the king? Sorry, that's a failure. He succeeds in taking out the Big Bad with one blow? Sorry, no, it didn't happen. I believe this is an equitable solution to character immunity on one side or total party kills on the other side of the spectrum. This announcement allows you, the player, an opportunity to play knowing the full choice available to you and allows me the ability to change the results as I see fit and have the ruling in the open, rather than behind the scenes fudging rolls.