Obviously a bit daft to generalise about any musical genre but if I were to pick one with a higher number of capably violent people I'd say Streetpunk (also known as oi) and if I had to pick a second it would be hardcore punk.
I actually grew up around people just like that. Point is, the music they listened to never mentioned violence in any shape or form yet they would happily beat up anyone they could get their hands on for any made up reason they could think of - like being a metalhead or just wearing the wrong color pants.
So you've added the criteria per capita; I still think you'd find that the gang milieus that gangsta rap (in its various forms) is closely associated with vastly overshadows any isolated cases of Nödtveidt or Varg-wannabes there still might be out there. Black Metal is also one of the most popular subgenres of metal nowadays, so the per capita argument doesn't really apply.
RAC/White Power-music perhaps, but then only because the fanbase of this music is so embarrassingly small.