Well, this isn't a very hard thought out idea, but here goes. I believe Sammael killed Asmodean. He certainly had motive enough(keep rand from learning too much). And he could of had knowledge. You see, the mentioned too little of Pevin for it to be insignificant. He was the bannerman, and he just happened to have a scare like Sammael's. Think. Would Sammael lose the scar, even in a disguise? It wouldn't be like him. He is obsessed with it. So, if he managed to get that close to Rand, he would know about Asmodean.(also, he wouldn't try to kill Rand. He would have no reason...yet).