In my opinion, heavy metal is an art form, and like all art, some of it is good, some of it is awful. If you like it, then enjoy it for what it is.
Some kids may get the wrong idea from metal, though, and that is why one needs to see it from a specific perspective. This is what I did. A few years back, my daughter tried to shock me (like many teens try on their parents) by saying that she was really getting into Marilyn Manson. I shocked her instead, saying that I really liked him, too. She wanted to know why, so I told her. Brian Warner, Manson’s real name, is a marketing genius. He takes on an offensive name, mutilates himself, makes really hard-core music (I personally do like his version of Sweet Dreams!), does all of these publicity stunts like claiming to worship the devil and ripping up Bibles, and what does he get? Horrified parents and fundamentalist Protestants getting all worked up over him. And the end result? He laughs all the way to the bank. Crestfallen, my daughter gave up on him immediately after. It just wasn’t cool anymore!
Believe me, if Manson actually encountered the devil as he really is, he’d whimper and beg God for mercy. Same with Ozzy Osborne, whom parents of my teen generation were all worked up about. Today, the man who was so feared is now everyone’s favorite daft uncle.
So no, metal by itself is not of the devil. I am sure that people could commit major sin by listening to Barry White or The Carpenters, nevermind Black Sabbath. In the end, it is marketing that plays to the masses and makes a boatload of money off of them.
Enjoy the music it if that is your thing.