That RJ considers it naturally obvious that the Shadow's top bounty hunter & assassin, who RJ even nicknamed Slayer and who made other similar attacks also on renegades in the series before, killed Asmo as well.

At the time of Asmo's death Graendal wasn't more obvious than Sammael. They were similarily obvious (or not obvious); if anything, Sammael might even have been a bit more obvious, since he actually stated that he would kill Asmo in the beginning of TFoH.