I generated a small world and let it only get 6 years of history (the only way i can get less than thousands of goblins in dark fortress apparently) so i could play a dark fortress assault instead of just lagging my system trying to just move around those building.
I entered the big building at night, every trolls and goblins were sleeping, though it would have surely be unecessary as with how buggy reaction/emotion are, you can usually freely decimate whole civilisation alone without any individual trying to even defend itself.
After letting dozen of trolls and goblin corpses behind, i finally reached the top of the fortress and found that guy :
The master of the goblin civ, a demon named :
A demon giant kangaroo with pink hairs ! wow
And so the silly stabing and slashing started with my copper 2 handed sword, a few hundred of hits landed, bruising, severing nerve, cutting etc... in the head, that wasn't making much sense, but sometime DF2014 damage system can be odd, i did 20 successful hits in the head of a goblin sleeping without managing to kill the poor thing (superior strength and multigrasped the 2 handed sword, i had nothing else in hands) after i grabbed his copper helmet and removed it to be sure there wouldn't be a festival of "deflected".
As despite the huge number of his the demon wasn't still dead (probably the "very fat" was in the head too), i decided to try hit elsewhere
But still the thing wasn't going down anytime, it even seemed bored by talking to me, i was wondering if my character was not going to pass out from exhaustation when for reason i didn't understand, suddenly the overly passive (i hope those reaction/emotion bug will be fixed in next release, that's the worst "gameplay" bug in my opinion, destroy the immersion of the game) demon decided it was enough and
Ouch, that must hurt a bit.
So it looks like the overly passive and lack or reaction/emotion bug that afflict a NPC will stop after you hit them more than a hundred of time.
Probably why i didn't see any npc reacting before, because that's the 1st guy since i play the new version that survived without problems more than a hundred of hits in the head.