“What is evil about Metallica? Their sound?”
This is something of which it is almost impossible to persuade lovers of evil music, so I’m not going to try.
One thing I might recommend is going back to the early days of rock (dancing) and roll (f*cking) and revisiting the evils that were predicted to result from it. And yes, that is what rock and roll means, and always has.
Find one case in which these dire predictions turned out to be false. Start with the fact that I can write “f*cking” in a public place without being arrested for it as a case in point.
We need to make sure these kids are not exposed to the murder and adultery of Verdi and Mozart operas, much less the demonic paganism of Wagner.