Try to think with Robert Jordan head: you want to introduce a character disguised as Rand's friend but in truth one of the forsaken,but you have introduced another character before, a character that knows all about the forsaken and that can easily unmask the spy: how do you think you can save your new character? Killing Asmodean! But this create some problems because we know the major suspect is Demandred but he has not killed Asmo as clearly said in "Lord of chaos" prologue. This is the second reason because Demandred couldn't be Taim, the first is that Jordan gives us too many clues that indicates Demandred as Taim. Then I don't know Asmodean assassin identity, but only because I don't know Taim real identity, found it we'll find the killer. P.S He cannot be Fain because Asmo never knew him having being freed after Fain abbandoned the DO way.