I don't have any definite theories as to who actually killedAsmodean, but I would like to mention something that occurred to me. Moridin or one of the *'gars _could_ have been recognized by him, in the same way that Aran'gar was recognized by Delana in LoC-- through his dreams. The evidence tends to show that most or all of the Forsaken were Dreamwalkers, and even if he had warded his dreams, Lanfear mentioned once that she could break through Rand's warding, so it's obviously possible.