My theory is that the murderer is none other than Shaidar Haran, ie the Dark One's little buddy or perhaps the strongest manifestation the Dark One can pull. Who else would frighten Asmodean so much. Certainly not the other Forsaken. He would try to whine and cajole them. Asmodean knew he was dead the second he laid eyes on his killer. Who else would slay him instantaneously for failure. The Forsaken would keep him around, he was no real threat to them. It was the Dark One in the Billiard Room with the Knife.