That RJ considers it in a way naturally obvious that the Shadow's top bounty hunter & assassin, who RJ even nicknamed Slayer, who certainly knows the Caemlyn Palace best of all suspects 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.

Of course one shouldn't forget that the comment was a bit tongue-in-cheek. BS said for example that he wouldn't have figured it out.