This is a Game Dev created problem not other players.

You are aware there are certain Motifs/Furnishing Plans and Furniture's that can only be obtained from Pickpocketing/killing certain NPC's/Races/ NPC's in certain maps right?

Now I am not sure if the specific NPC that you are talking about is one of these. But the way you are putting it is that players being able to kill NPC's is a problem in itself. Outside of very specific situations (such as people needing to kill the exact same NPC for quests/achievements) this is not a fair thing to say.

I say that because the Justice System/Stealing/Pickpocketing system is a whole game system in itself and people don't realise it goes sort of deep. I guess most people just think something like: "oh you can kill these guys for some valuable loot to sell". It goes a little deeper than that is all I am saying.

Anywho, I hope your message gets through and the dev's make this said NPC unkill-able

Good Luck.