Well, I've never liked "heavy metal" or any other metal music, but I know some metal fans who are actually pretty cool people.
It is a very complex issue here because on the one hand you have an arsonist who can be just revealing a facet of a very twisted mind; then we have a crime agianst a very particular demographic group, which be be typified as racist/hate crime, and then a crime against a religious institution.
As I have said many times, the fact that I do not believe in any dogma held by any organized religion does not mean that I can justify any agression against religious people or their house of prayer.
If I do not like them, I just ignore them (hoping that they would do the same with me).
The metal issue makes this more bizarre, as one of you mentioned, it gives the group a bad name (as if the mere assumptions people make about heavy metal fans were not enough).
And here is where I take issue with any "artistic" movement that allows dark and ignoble practices to be part of their identity.
I think that all extremes are negative and radical actions like this one do little to reduce tensions between those who think that religious is the only way and those who think that sanatic attitudes and actions are actually the only and better way.