for the "Who killed Asmodean?" question, let's analyze this:
Asmodean says: "You? No!" before dying. So it must be a Forsaken that he knows. (RJ says its obvious, so i'll go with the obvious of it being a Forsaken)
Ishamael: Dead, even if alive, he has a new body, so Asmodean wouldn't recognize him.
Demandred: LOC, Prologue, The First Message, pages 15-16, Lanfear has vanished without a trace, just as Asmodean did. He wouldn't lie about this to the Dark One at Shayol Ghul
Semirhage: LOC, Chapter 6, Threads Woven of Shadow, 141, Sem thinks Asmodean. A traitor, and so doomed but he really had vanished... and later If the Great Lord moved her here secretly, might he not be moving Moghedien or Lanfear, or even Asmodean?
Obviously does not know if Asmodean is dead or not
Moghedien: Under the effects of forkroot, and/or prisoner of Nynaeve with the a'dam
Aran'gar + Osan'gar: See Ishamael
Mesaana: Hadn't been mentioned up to this point
Shaidar Hadran: Wouldn't have been recognized by Asmodean
Sammael: LOC, Chapter 6, Threads Woven of Shadow, 133, we have big evidence "Rumors! Lanfear has been aiding al Thor since the beginning, if you [Graendal] ask me. I would have had his head in the Stone of Tear except that someone sent Myrddraal and Trollocs to save him! That was Lanfear; I am certain. Im done with her. The next time I see her, Ill kill her! And why would he kill Asmodean? I would if I could find him, but he has gone over to al Thor. Hes teaching him!
Sammael probably isn't lying here because of the pure fury he's in; he's known to lose himself in his anger
Lanfear: Prisoner of the Finns.
so that leaves...Graendal.
There ya go, Graendal killed Asmodeon