It's reported that RJ "emphasized OPPORTUNITY as the key to knowing Asmodean's killer, even over motive". Plus, he answered the question: "What criteria are most helpful for investigating Asmodean's murder, out of motive, means, timing?", with: "Timing is what you should focus on." (apparently not so much motive or means). It becomes a bit clearer through RJ's statement that "Padan Fain didn't [kill Asmo] because he wasn't in the right place at the right time. He would have if he had been in the right place at the right time, but he wasn't.

Although people repeatedly claim it, RJ never stated that it was "only a murder of opportunity" in the sense that Asmo by chance bumped into his murderer, i.e. a completely unplanned confrontation. The criteria "timing" wouldn't even make real sense then. It's definitely plausible that the killer found Asmo in the Caemlyn palace some minutes before and then followed him to end his life at a secluded place. The killer was obviously better prepared to kill someone than Asmo was to defend himself, considering the quickness of Asmo's death.

The killer managed to be in the right place at the right time to kill Asmo due to the right timing. This speaks for the Top assassin Slayer, who we often see spying on people everywhere, who might have gotten the general order to kill Asmo, if he finds him (but might also be in Caemlyn to find Fain or because of dealings with Rahvin), who knows the Caemlyn palace best of all suspects, and who can use TAR to appear in the right place at the right time to perform his assassinations out of thin air (Asmo didn't already see his killer, when he opened the door). Naturally the motive only comes second for an assassin, because he usually kills someone because he just wants to fulfill his orders, not because he personally wants the victim dead. The means aren't that important (it doesn't have to be balefire, for example); also an able non-channeler could have killed Asmo in this situation.

Contrary to that, it's not clear at all why timing or opportunity would be the keys to speak for Graendal more than for any other Forsaken, e.g. Sammael. Besides that, a Forsaken would naturally have a stronger motive to kill the rival Asmo. Usually the means (namely the ability to channel) are also used to support that Graendal must have done it, though apparently this isn't that important.