So I read through the journal at the entrance, and a paragraph really caught my attention:
All along, I was mistaken. All along, I was the blight upon Sithis and his dark name. Tonight, he spoke to me and again, and I learned of his displeasure. Again, I heard no words, but I knew the meaning. I was meant to take blood, to spill blood... but never to taste blood. My sanguine ways have offended my lord! I must cleanse myself of this filth. I must find a way
So.. Did I miss something, or is Sithis offended by vampirism? Is it "wrong" to be a vampire and a DB assassin who serves Sithis? There is Vicente Valtieri, and he doesn't seem to be worried about it..
Or maybe I've completely missed the point, and Greywyn was just crazy..