Connection between heavy metal and emos, bikers and the mentally ill?

This may sound prejudiced, so I want to say right away that the following is just my own experience, and I know that not everyone who listens to metal is an emo, a biker, or a psycho. On the contrary, I know a lot of "conforming people" who listen to metal. By that, I mean:

"Not many heavy metal listeners are emos (etc.), but many emos (etc.) are heavy metal listeners"

Somehow, all the emos, bikers (mostly Harley bikers and more Hells Angels-like than regular drivers), and people with mental illnesses (less so depression, schizophrenia, and anxiety disorders, but I encountered a particularly high proportion among borderline patients and patients with bipolar disorder) I've met so far are heavy metal listeners. In keeping with the cliché, they almost always included a pentagram, and occasionally a St. Peter's Cross.

The percentage of heavy metal listeners among such people seems to be immensely higher than among "people who conform to the norm"

My question now is whether there's a specific reason for this. None of the people I interviewed could tell me why they wore pentagrams and St. Peter's crosses, or they had no idea about Satanism or Christianity, but they said, "Satan is cool." When asked what they liked about metal, the answer was mostly, "I just like the music." A plausible reason. But what could be the connection between "people like that" and metal?

And as I said, I don't want to generalize, it just strikes me as very intense and I'm very interested in such phenomena