It's the violent ones that don't get the catharsis aspect of expression in hardcore or other intense forms of music subculture. The mosh was never about hurting someone else, it was a spontaneous, somehow-safe (as long as that guy didn't show up) expression of pent-up violent energies. At the end of the show, people go home feeling better about their lives and are probably less prone to violent outbursts in daily life. Meanwhile, it's safer on the books than football, the great American sport.
Then this archetype shows up and sees the violence and starts forming gangs and trying to restrict expression, and the violence happens. And the whole punk movement has always had to defend its own property rights, because there is a known public bias against the community. It always seems to happen when the movement as a whole gains mainstream attention. The media mischaracterizes it and the violent element crawls out looking for bloodsport.