Ok, so I have a couple of ideas for potentials, that I don't recall hearing much of as of yet. Let me start with the very last lines we read of Asmodean.

"He pulled open a small door, intending to find his way to the pantry. There should be some decent wine. One step and he stopped, the blood draining from his face. ""You? No!"" The word still hung in the air when death took him." -The Fires of Heaven, chapter 56 Glowing Embers, page 682 (hard cover copy)-

Now Mr. Jordan was famous for saying that the answer to the question should be clear even to the most casual observer. Most of us, (myself included) have decided he must have simply been having fun with us. The more I read these four lines though, I think he may indeed have said it straight as it is.

The explanation is quite simple really for the first possibility.

It was Moridin... What is death in the Old Tongue? Mordero/Moridin (loosely translated to death, or the grave). "The word still hung in the air when DEATH took him." I think this could be a direct reverse translated way of saying that Moridin killed him.

Now people can say this does not chronologically work right? Well we really don't know that. Ishmael could have been freshly brought back, or had been in the waiting for a while. How long was he free before the others, and managed to do what he did without anyone ever knowing he was free. I think he was brought back a short while before, and getting rid of Asmodean was his first order of business straight from the Great Lord of the Dark. No one really knows what the DO knows, or how much/when he gets his info (he has surprised a few Chosen with his knowledge at times). Moridin shows up using the True Power (thats why no one sensed any channeling) using an illusion weave to make himself appear as he did before his death. Flaming eyes, his the old clothes etc. Think of how surprised Asmodean would have been to open a door and see ol' Ishy standing right in front of him.

The other option could be Shaidar Haran. This was more somthing off the top of my head, but he channels "something" (TP also??) at different times in the books. I don't think any of the Chosen had seen him up to this point though, so it was more of a "fit the criteria to be able to do it kind of thing". Appear in a dark pantry with Myddraal power, check. He could cut Asmodean off from the source, and kill by hand or with TP, and no would sense any channeling, check. And the line, "One step and he stopped, the blood draining from his face." Could be Asmodean looking up into the eyeless gaze and feeling the fear most everyone else does.

Anyway, thats all I got on this for now. I hope I have given at least a few people a little something new to think on while we wait for the answer.