ACK...I am level 56 and just to try this out, just for fun, I tried to kill the Dorfs DEN and the others in those little houses by the water in BB zone , I am KOS to them.
Well as I cast my DD Scor and even Calefaction, it says my "Spell wont take Hold"..............the Dorfs wont even try to attack me and I cannot even try to melee them.
What is going on?

You can't kill them anymore.

I never understood why they made NPC's like these invincible. I mean, its not like they are guards for a newbie zone.

Are they used in any quests? If so killing 'em would be annoying to anybody trying to do the quest. Note that 1) it's conjecture as I've no idea if they are or not 2) I do realize that at this point in the game's life it's unlikely they're for any "important" quest and 3) not like quest mobs haven't been killable before.

Faction war exploit.