The pathway to the chest for his treasure is to the right of the spiral staircase, on the eastern most part of the south facing wall iirc.
I actually ended up reloading it to kill him simply for the satisfaction of doing so, since you also get the tunic if you kill him and the rest would've just been sold. Mind, I was playing on Hard at the time, and the fight was actually pretty easy (Mage just spammed slow on him iirc after he casted SoA).