Come on people, it was blatantly obvious that Slayer was the killer.

Let's look at the situation...

Asmo was ASSASSINATED...by the shadow...hmm, who are we told is the shadows assassin? (not a trick question).

See there were chosen with motive, be it Lanfear, grendal, whoever...even if they were going to kill him, they would have used Slayer to do it. Throughout the entire series the chosen are a bunch of cowards. The only time they fight is when cornered to do so. ie Rand attacking them, or Moridin bullying them to attack. The only time I can remember them actually doing something themselves is when they launch that attack on Rand in the watch tower thing. They also fled before following up on the attack. Time and time again they use shadowspawn and DF's to do their dirty work.

Point is Slayer killed Asmo, it was his job. Who ordered it? I have no clue. There is no evidence that Slayer couldn't have done it. It is however a stretch that anyone else could have pulled off this stealthy assassination.