Granted, lack of motive was always a puzzle for me - Edit 1
Before modification by Skeeve the Great at 19/10/2010 05:17:49 AM
If one goes on the basis of what Jordan said, there is nothing outlandish about it. He said it was an "attack of opportunity" (effectively ruling out Slayer, to my mind) and that we had enough information to figure it out by the end of Book 5. The obvious answer is Lanfear, given how often she threatened to kill Asmodean if he betrayed her, the fact she knew where he was and the fact that she had wishes with the Finns (as I have pointed out on countless occasions).
But Asmodean didn't betray her - he taught Rand as was expected. Even when Rand used Lews Therin's knowledge against her, she reasoned that Asmodean was gossiping about her but dismissed it quickly. Cyndane has also made it clear that she would like to directly go after Rand, but Moridin has repeatedly stayed her hand. It doesn't make sense to say that Cyndane would instead choose the lesser prize and go for Asmodean.
Sammael is too direct to sneak around and take out Asmodean, with more than one hint that he likes to be the center of attention and do things openly. Graendal is surreptitious and complex. She's said she wouldn't go after Rand directly and has been made aware of the fact that Rand is being taught by a Forsaken. With the death of Rahvin and Lanfear, it took every little for her to act. We know that she can act if she wants to...
The only thing that was pointed for me to Lanfear is a lot of Lanfear scenes in FoH, her constant threats to Asmodean and her out-of-control rage at the docks. But yes, the motive is the weak point here.