Making every NPC killable & and letting the player know that a specific quest was screwed because of that via message will be very nice.
If my character is more powerful than Eliminster(lvl 28 mage in 2ed) there is no reason a random guy in a thief guild or Marchor Harpell to be able to slaughter my party just to prevent quest breaking or ridiculous balance issues.
What's the point of them slaughtering my party? I can't continue playing either way. (either I screw up the quest or I die) if that's the case, killing those NPC will at least feel satisfactory.