Strong generalization of who listens to metal music. I know numerous conservatives who listen to Metallica (one of my favorite groups). Heck, a 60+ year old women at my old church played Metallica's "Fade to Black" on her accoustic guitar during a Bible Study.
Agreed; going with that, you probably couldn’t separate the good bands from the terrible ones.
I listen to stuff like Iron Maiden, Helloween, Pagan’s Mind, Threshold, Shadow Gallery, and things of that nature, the progressive/power metal side of things.
You’re probably thinking of the bands that get huge play on radio and TV.