I'd like to see a study like this done on members of the Norwegian black metal scene from the early 90s. Because I still can't figure out what the burnings and stuff were about (aside from paganism, I guess), and “TROO” metalheads sound pretty crazy even compared to all the other ‘alternative’ culture aficionados. But generally, of course, this experiment seems like just one bit of a proper comprehensive study.
Norway didn't really have much of that. It was (and is) a really safe country, very stable, religion is mostly pretty bland non-offensive Protestantism, most people maintain very ordinary non-spectacular lifestyles.
The 2nd wave black metallers felt completely suffocated by this quiet peaceful existence, so they rebelled against the peace and quiet and tolerance, and took teenage rebellion to absolute extremes, doing whatever they could to frighten the ordinary people.
There were probably also other issues wrapped up in it, I think Dead (Mayhem's lead singer, who killed himself) was depressed or maybe had even deeper issues. These guys felt like outcasts in their own homes, so they acted out.
TROO or TRVE metalheads aren't just in black metal, although I think they are more prevalent there. We have them in every genre, from death to folk to power metal and beyond. There's a drive for some people to identify themselves by the things they like and even more by the things they don't like. Some people just get really wrapped up in something they care a lot about, and music affects us on a very primal level.
Here's a great Defcon talk from Neil Fallon (lead singer of Clutch) about how music tugs at our emotions: https://www.youtube.com/watch?v=cVx5Y1OE-KE