Well that's if you are on that part of the chain that is.
Seems like you cannot kill him for runes and go back and get his Arm after you pick up the quest.
IIRC Lei Shen let you get his Heart on a second kill. :/
Guess I'll have to wait another week now, will finish my runes tonight but killed Blackhand just to see it.. .
Edit: This is to get on to the follower section.