But I certainly think he fits the clues from the books and what RJ said best.

It was reported that RJ said Asmo is "roadkill". In slang that simply means - and I cite the free dictionary, but there are similar definitions elsewhere, too - : "One that has failed or been defeated and is no longer worthy of consideration." It's certainly not meant in its literal meaning, namely that Asmo is "an animal killed by being struck by a motor vehicle", since that wouldn't make much sense. Meriam Webster even defines roadkill as "one that falls victim to intense competition <political roadkill>", which would support the idea of a fellow Forsaken sending a killer to assassinate the rival Asmo.

"Roadkill" most likely means here that Asmo wasn't needed anymore in the story and thus the author let him die in some way, but not that the murder wasn't planned at all by the killer, and that it was basically a complete accident.

RJ also said that "timing is what you should focus on" and it's reported "he emphasized OPPORTUNITY as the key to knowing Asmodean's killer, even over motive (which got a mention)". This makes clear that someone saw an opportunity to kill Asmo, and managed it to get to the right place in time. This is further supported by 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."

This makes clear that the killer managed to use the opportunity to kill Asmo in time, likely because the killer figured where Asmo would go (and if Asmo hadn't opened the small door, the killer had simply waited for another good opportunity).

Plus, that even Fain would have succeeded to kill Asmo, shows that certainly Slayer could manage it as well.