He told us Fain would have had the will/motive to kill Asmodean if he had an opportunity.

Fain could kill a Forsaken in the right circumstances with his dagger - he got lucky and nearly killed Rand by sneaking on him, but in the circumstances of Asmodean's murder, the only reason why he may have succeeded is because Asmodean didn't know Padan Fain but Fain can sense the Shadow. Still, this would have been a close thing, as Asmodean can channel and would have the second he realized this man intended to harm him.

Jordan was merely saying "Fain had a motive, but not the opportunity". That's all he was saying.

There's nothing in that quote that favour or disfavour Slayer. Jordan was ruling out Fain as a suspect in a humourous way (he was given a list of suspects and asked to remove one person from it), don't go and twist it as if the question had been "could a non-channeller like Fain kill Asmodean?".