It's probably way too late to be skeptical about this, but do we actually have any evidence that Sephiroth was the shooter besides an article from TheDailyBeast? Which isn't a particularly reputable news site anyway? One of their reporters was probably googling William Atchison looking for scoops, and found this thread and decided to report on it.
Like, I was originally on-board because I found one of Plaz's old IP addresses in a thread and I did a reverse DNS search on it and saw that it was in the Albuquerque area, but since then we've realized a.) it wasn't Plaz, and b.) the IP address probably decayed and now matches an incorrect location. Not only that, but we've changed the whole narrative to it being Poiuyt who was actually posing as Plaz on his Sephiroth account, but I've heard from several other users that Poiuyt lived in Canada.
So effectively the only hard-evidence that remains here is the fact that TomTheGeek posted the name 'William Atchison' on a thread where he was trying to dox Poiuyt/Plaz, but again that's not necessarily an impossible coincidence considering the amount of private investigating that went on a few years back.
Am I up-to-date on everything or is there some kind of additional evidence that I'm missing/