I havnt read the discussion or any of the other replys yet but its not hard to dismiss this as absurd. First off dont get me wrong, this style of music effects certain people in weird ways. Like when I heard about some guy that won a court case that said he was aloud to listen to metal at his job because he claimed to be addicted to it. So of course certain violent, disturbed people may react violently to the agressive aspect of the music.
To make the claim that "evil ", "satanic", or plain aggressive metal music alone will cause violence is a little naieve(sp?). If you cant step back and look at the world that we live in, especially in America, and see that we are constantly bombarded with and increasingly desensitized to violence than you might as well continue to blame metal. I have even been thinking recently about how many of my favorite video games focus on violence guns and other weapons. Then how many games have nothing to do with violence. I dont play that many non-violent ones. This is true of almost everything else we view through different media. On top of that the current administration of the most powerful nation in the world, the one that many blindy trust without question, would rather use violent force over peaceful diplomatic resolution. This also goes hand in hand with the militarization of our police force, and other goverment backed or privately owned thugs. Those who constantly expose honest taxpaying citizens to the presence of big guns to the point where we just see it as a common practice.
I apologize for going on and on and I could go a lot deeper but long story short as far as main causes of violence go metal is the last thing to worry about. If anything I would have to say that metal deters violence in most people by creating a positive outlet to deal with the emotions most commonly associated with violent behavior.