Immortal NPCs are completely against the whole promise of freedom, action-consequence and ''immersion'' that Bethesda vaunts so much. I disagree with several of their design approaches, but none come as close to being as outright bad as this one. Having the Blades and Greybeards immortal until they are no longer needed? Fine, the plot has to advance somehow (and even then, I would suggest that the Greybeards just use the Voice to instantly kill the player rather than being immortal, but whatever). Anybody else should be completwly free to kill. Oh, it makes the town angry? Oh it breaks a quest? Oh, it makes X faction hate me forever? GOOD. That's called action and consequences. This is what makes an RPG. Not how many shapes of nose my character can potentially sport.
If the issue is dragons or other nasties killing them, OK, just make them invincible unless killed by the player. Would still be bizzare to see them just kneel while the big lizard attempts to burn a random civilian to a crisp, but at least my killing sprees have some impact on the world beyond a small bounty and a stern look from the guards.
And yeah, the camp leaders are the very worst. Having my super-badass, dragon-commanding, giant-massacring, Deadric armor clad self run from a run of the mill Stormcloak seargent because I cannot kill him is absolutely stupid, and there's no excuses in all the world for that. It kill my ''immersion'' far more than not being able to pick up random junk would.