I wouldn't say unkillable, but (as most of them are) unlootable. All NPC's should be unlootable, so that Caravan farming, or even quest giver-farming will stop.
basically having killable merchants gives griefers a chance to basically tank the player crafting economy (which doesent affect them in the slightest becuase they're either 1chr pvp builds or suicide bombers) so they dont give a shit.
Ok since people dont seem to get the idea heres the issue, killing a merchant serves NO purpose and if your only doing it to be a dick then you honestly shouldnt be playing the game, go maim puppies or do whatever it is sadistic people do.
Afaik important NPC are unlootable, unstealable and killing them gives you nothing. But it certainly is a way how to raid towns and act like bad guys. I would certainly let everyone killable. If you can't defend them then be ready to trade only in inner city...
But what's the point of killing them if you get killed right after it? Dead guys aren't bad or good, they are just dead. Basically everything in the game should have a purpose, if killing some merchant was part of a quest and there was a way to get away with it alive then it would be completely acceptable.
would someone explain to me why making them unkillable isnt the solution instead of just saying it isnt. and to people who thinks that this would kill immersion thats just a flimsy excuse, in all actuality immersion is dead the minute the merchant comes back to life or you see some go LOL NOOB!!!111 or someone exploits the game etc etc, so stop trying to defend another way to grief players.