I have never registered or posted at ATS, though I have looked over a lot of threads. I don't like their format-- it is kind of awkward to read. I'm pretty sure I've seen posts from "erad3" in the past, when I've gone to ATS. But I can't find them now.
Of course, it's not clear if Laughner really did this, or if it was a frame-up. The posts he started are consistent with the Youtube videos he supposedly put up, fwiw. The big worry is this is yet another way to slur/slam/scapegoat the conspiracy community, and that there will be a call for more regulation of conspiracy websites.