This was the book in which Rand became or lost estranged pretty much everyone he had personal ties with or who could counsel him or whom he could trust: Mat who evaded him, Egwene, Moiraine, Lan etc.m- and the person he confided the most to was his Forsaken teacher, whom he lost at the end as well. From there, he was virtually on his own among strangers, and several new dubious characters where about to enter the stage, and we would see them and make our opinion of them mostly through Rand's POV only - people like Bashere, Taim etc., agreing or disagreeing with Rand about the trust or mistrust he felt for each of those. Rand's POVs were conveying to us his increasing paranoia, which in part we were meant to share, which could only work if we were missing information about stuff that looked relevant but wasn't necessarily important, hiding among the red herrings a few of the important things to notice.

RJ handed us extra info Rand didn't have: Asmodean had not fled, he was murdered. Not revealing why and how he was killed created the feeling there was a hidden plot ongoing Rand was not seeing, obsessed with his notion that Asmodean had fled. This also contrasted Rand's decision to trust Taim, whom nearly no reader trusted from his first scene (nor were we ever supposed to trust Taim, Jordan wove around him the same themes of jealousy and scorn of Rand he had introduced for Demandred, then we the Taimandred phenomenon started he sort of backed off some, probably guessing he had overdone it if people associated Demandred and Taim so fast, to the point of believing they were one and the same).

Of course, it's the most obvious suspect who turned out to have killed Asmodean. Of course the murder actually was "irrelevant". It had played it's role, of getting rid of Asmodean, and augmenting the paranoia about plots around Rand. It had no bearing whatsoever of the rest of the story and what happened really wasn't all that interesting, which was probably why RJ originally intended never to return to this and spelled out who it was.

The rest, it is the readers and the internet, that turned a minor story point into this large mystery. That's only after that happened that RJ started having fun with this... not his fault if readers insisted so much on questioning him about this so-called mystery, mind you. He was actually a good sport to have included enough clues to solve the mystery by eliminating the Chosen who didn't kill Asmodean. All those crazy theories about Lanfear, Slayer, Aviendha, Mat, Moiraine... RJ's not to blame for making a huge deal like this out of it. When the writer keeps insisting that he doesn't intend to reveal something, the obvious conclusion, one many argued about for years, is that the solution must be irrevelant to the greater story, and not interesting enough to include in the books... But readers kept insisting, and RJ gave in - and so there it is, in all it's irrelevance, obviousness and boring glory. If RJ were still here, he could say "well, now you see why I thought it more interesting never to solve this".