Actually RJ didn't say that it was an "opportunity kill" or a "murder of opportunity" as people often claim, but that we should rather concentrate on the question, who had the opportunity to kill Asmo. It was just reported (!) that RJ "emphasized OPPORTUNITY as the key to knowing Asmodean's killer, even over motive (which got a mention)."

This fits a killer like Slayer, who probably waited some time for a good opportunity to strike at Asmo alone (e.g. in a deserted palace, he knows very well as Luc), but doesn't have a strong motive (except for fulfilling an order). It's not clear at all why opportunity would point to Graendal, since the plan of the allied Forsaken in TFoH was never to go to Caemlyn.