None of Rand's Aiel who isn't a darkfriend would have killed Asmodean and not go straight to Rand and confess he had toh...

That's how the Aiel do it. They do what they must, and then pay the price.

Aviendha, Amys, Rhuarc and co., they're all excluded as suspect (unless one of them is a DF). They would have done exactly like that warrior who felt compel to kill a Cairhienin he perceived was insulting the Aiel. He killed him, and he went straight to Rhuarc/Berelain and told them he had toh toward Rand.

If Sorilea had felt she must kill Semirhage, she would have done it and go straight to admit it to Rand.

And that's what any Aiel would have done with Asmodean. He/she would never have made the body disappear and hide from Rand what happened, or even lied when he asked about Natael (and he did ask the Aiel if they knew anything) That'd be completely dishonourable in their eyes.

Don't forget also that the murder is one of opportunity. Mere seconds before he opened the door, Asmodean himself still had no idea he was going there, and he had been walking without an aim for minutes. No one coud have any idea where he was going. No one was waiting in ambush, he surprised someone who would kill him in the room/corridor the door opened to. That person might be someone who was there to kill Asmodean (as far fetched as it sounds to me, but then two strong ta'veren were near), but that doesn't change the fact Asmodean can only have bumped into his killer by accident. So stealth etc. - those are not issues.

As for motives.. they are limited to those of people who would have killed Asmodean if he surprised them in that location. The Forsaken would have done it, one their servants might have seized the opportunity... but non-DF in Rand's entourage? I seriously doubt that, especially no one who intended not to get caught. Non-channellers wouldn't have been stupid enough to attack a Forsaken standing right in front of them, anyway. Only Lanfear and Rand (and seemingly Moiraine) knew that Asmodean was shielded.

The "motive" is simple enough: Asmodean would have killed or capture his killer if she/he let him.

The real questions is what a person Asmodean would kill/capture was doing in the palace, and in that room/corridor where he surprised her/him. Whoever one think killed Asmodean, whether Graendal or another Forsaken, Slayer etc. those are the relevant questions. That, and who had the means to kill him in a few seconds, without Asmodean having time to channel back. Finally, who had the means to make the body disappear.

Finally, Jordan implied Wilson could have figured it out by reading more closely the scene. We've dissected the scene long enough, there's virtually nothing in there, except to exclude people and circumstances (it can't be an ambush, it happened instants after Asmodean saw his killer etc.) The only clue is that Asmodean thought that door lead to a pantry, and why would he think that. A pantry in a palace is a room for the servants, where they wait for orders and keep a few things at hands the masters ask for often (stuff to make tea, for example). We saw one in TGS, Nynaeve went straight to the pantry when she wanted to find a Chandmar servant on duty who could fetch the head servant for her. Was that door in the corridor really leading to a pantry? Maybe not, but would make Asmodean think so? Well.. a door for the servants's use would be smaller and more simple than all the other doors in the corridor. We are most likely looking for someone who had a motive to visit a servant area, with or without a pantry, in the palace hours after Rand killed Rahvin. The most likely motive, for someone who also fits the "Asmodean would kill her/him on sight" factor is information, information about Rand or his entourage (that of course could include Natael). Whoever killed Asmodean, Graendal, Slayer etc., they were most likely trying to find information from the servants (eg: Graendal might have been there to get the same Nynaeve wanted: find one of the very few servants still around to fetch her Renee Harfor whom she would compel to order the servants to spy on Rand and report to her. eg: Slayer pretending to be a minor noble (he has the look to pull it off), to find out where Natael or Rand would sleep that night, so he could surprise his target at night.. that's a much safer way than roam a palace full of Aiel to track Rand, Natael or anyone else down) .

In any case, if the association a servant area/pantry is one of the details from the scenes Jordan thought we would pick on, then it points to Graendal, the only Forsaken that has been associated to servants in that book, and there's even a reversed reference to using them as spies (she tells the others not to fear speaking in front of her servants)