There are lots of sub-genres to metal music. Instead of attempting to explain them all, I'll present the process I use when categorizing metal:
To the untrained ear, it's easy to lump everything on the right side of this chart into one big, ugly, satan worshiping mess. For those who enjoy metal, it's equally easy to get sucked in to how awesome a band sounds and forget to think about the message they're presenting.
For quite a few years, my favorite band was "Lamb of God". This name is not a positive reference to Jesus. Their sound is phenomenal, but ultimately I had to take their albums (and those of a few other bands) off of my shelf. Since then, I've decided to maintain a focus on the question
Does this band or its music present an ideology which explicitly runs counter to my faith and worldview?I imagine myself having a conversation with God. He asks
Why do you spend so much time listening to __________?
For a lot of secular music, I'd feel comfortable with my answers:
I like the high energy of their percussion
I think Norse mythology is really interesting
But for music which explicitly advocates an anti-Christian view, I feel like I'm insulting God by listening to it, which pushed it across the line.
I know I'd be insulted if my son listened to music about how awful I was.