From his most recent, pre-arrest feed:
One way of understanding this in terms of the fearsome internet is as a descendant of the digital terrorist theme. That reaches back into the 1990s (remember militias?), then advances to the 2000s' e-Qaeda meme.
Another approach concerns the building narrative about Twitter as a scary place (or service, or object). We've already noted some examples: scary people following you, or whom you can follow; a suicide threat; "ghost Twitterers" (authorship, not ectoplasm). All of these are classic scary-technology themes: stalking, suicide, dubious authorship, communications from the dead. Now we can add e-terror. No one theme has emerged as the leading fearsome-Twitter meme, so we're probably seeing a gestation period now. Perhaps this summer will see some major meme emerge.