Islamic terrorist? Unlikely, only because they have such a dismal record of screwups with ordinary things. He would have blundered, or been caught, or been in a standoff with police by now. If there's an Islamic terrorist with this much self-control, that would be a really scary thing. Plus, unclaimed terrorist acts are of limited utility.
Teen goth? Unlikely, for some of the same reasons as above. Generally has a suicide fantasy.
White supremacist? Pointless. These guys don't attack, they defend.
Genius fantasy type? Most probable, like the Unabomber.
Here's an exception to that rule (if it is one): Joseph Paul Franklin.