I think it was Sammuel or Graendal. It could have been Lanfear but lets remember that the door she went through is the same door that Mat went through and he was there only a few hours but in the regular world it was a few days, so Lanfear really didn't have much time to get out of throught the door and kill Asmodean In order to support the theory of Sammuel and graendal when Rand was chasing Rahvin he passed 2 servants, one male, one female. They were Sammuel and Graendal! After Rahvin was killed the book said that the servants had fled and didn't come back for a few hours yet those servants didn't leave. Also Graendal and Sammuel might have known about Rands coming by the fact that they were in League with Rahvin and he new that Rand was coming. i think that the actual killer of Asmodean was only one of the two but Sammuel talks with Graendal makes it seem that he didn't know about Asmodeans death first hand. And Graendal went to see the DO sometime that we didn't hear about first hand. So that's what I think. Please try to change my mind.