Since Tamyrlin says that it was someone that had already been introduced in to the story, it had to be an Aiel. Why you ask? We can rule out the Forsaken because in COS Moghedien say "The True Power was far more addictive than the One Power and that no one she knows has a will strong enough to resist using the True Power." So none of the forsaken use it. Without the True Power, Rand or Aviendha would have felt them channeling. We can rule out Shaidar Haran, Moridin, Osan'gar, Aran'gar and a Gholam because at this time we don't know that they exist. The Wise One can be ruled out because Rand did not take any to Caemlyn and they don't know how to make a Gateway. Of the Aiel that we know that went with Rand, Enaila and Somara where with rand at the time. Bael was hunting down the last of the Trollocs. Aviendha was with Mat and a guard of maidens under Sulin in the courtyard. Lamelle was killed in the attack. Pevin was also killed during the attack. The Servants and Lords of Andor can be ruled out because Asmodean would not know them. Padan Fain has not been to Shayol Ghul after the Dark One distilled his soul and at meetings, DarkFriends hide their true identity so one again Asmodean would not know him. The only possibility left is one of the maiden, who at one time was part of Rand Honor Guard. We know that an Aiel could kill him because Semirhage says in book six " If a kitchen girl puts a knife in your back are you less dead than if you fall in a sha'je duel at Qal?". Since only a darkfreind would know who he was, he would be surprised to have one he knew kill him. I believe that it may have been Liah who killed him.