Seriously.
Have this single goblin thief on my map after a reclaim, and a few goblin warriors. All were listed as friendly, cos of reclaim.
After while of trying to get the fort up and running again, I decide that these goblins need to go. So I send my woodcutter/axedorf to kill them, one by one. The soldiers go down easy. The thief decides he isn't going to put up with this kind of racism, and promptly stabs the woodcutter in the face.
Thief : 1
Fortress : 0
I'm a lil shocked, but not surprised, RNG has never been my friend in combat. Next migrant wave, I get a competant axedorf. I assign him the bronze battle axe and armour. I send him to the thief. Thief looks up from twiddling his thumbs in the middle of the forest to stab the dorf in the face, and sits back down again.
Theif : 2
Fortress : 0
I'm beginning to get more than a little annoyed now. I check the combat logs for both past attempts, both times he one shot's them in the head with his copper large dagger. Irritating, but this is DF after all. Next migrant wave, I get both an axedorf, and a swordsdorf. "Brilliant", I think, "they can multi-team him". I assign them both armour and weapons, and station them a fair way away so they can get prepared. Then I get interrupted by this message:
Urist McMoron, Swordsdorf, cancels pickup equipment, interrupted by Goblin Thief
Sigh. I zoom to it, just in time to see him get one shotted too.
Thief : 3
Fortress : 0
Great. Apparently he decided he wanted the sword that was right next to the thief. Nice one. I think, "it's ok, the great axedorf will end him". The axedorf runs to the station, sees the thief and mounts a charge. Wielding some sweet pods. /sigh. Guess what happens next?
Thief : 4
Fortress : 0
"...Right", I think. "Balls to it. I'll leave him in his little forest retreat, and he'll leave me alone". He does. For awhile, at least until haulers start trying to grab stuff thats lying around to put in the refuse stockpile. He gives chase to a few, but they eventually manage to escape. A miner then does the same, and gets chased. This time Thief-y McThieferson isn't giving up so easy. He chases this miner half way round the map. Eventually the miner counters and fractures the goblin's lung. "Hurray!", I think, "he's got to die now!" Goblin returns to his hidey-hole, and Urist McLuckyPick runs back into the comfort of the fort.
Thief : 4
Fortress : 0.5
But he gets better. The red wound to his torso and lung go yellow, then light brown.
Thief : 4
Fortress : 0
He starts back to his old tricks again. This time he chases a craftsdorf across the river bridge (over a cage trap, I might add) and into the wagon area. Haulers shit themselves, and an old trade liason (still on map, reclaim, remember) decides it's time to become a hero, charging at the goblin. Yeah, same old story. Now he's terrorising the entrance to my fort and there's nothing I can do.
/tl:dr, Why the crap are large daggers so bloody deadly?!
EDIT: The thief pounced on a craftsdorf and began perforating him repeatedly. Caravan guard hammerdorf nearby saunters over, casually, embeds his warhammer into the thief's brain, and saunters off again. Crisis over. Still, I hate large daggers.