I don't listen to Metal unless I'm very angry, and it takes a lot to get me very angry, most of the time. Most people that I know that listen to metal are alright, but tend to be a bit more.... aggressive. I'm not saying they're "I'm-gonna-smash-you-in-the-mouth-if-you-look-at-me" aggressive, but they are more aggressive than people who don't listen to metal. Heavy Metal doesn't cause aggression, but it attracts fans who are aggressive. Games, FPS's especially, DON'T CAUSE violence but they do ATTRACT the already somewhat aggressive fan.
And that's why I believe a lot of gamers like death metal. Then again, judging by the avatars I've seen in TF2 and on Forums, a lot of gamers also like anime, which is decidedly NOT metal in any way shape or form (well, at least, that's what I can tell. There's probably some extremely violent anime, but from what I see people watching or selling in the stores, it's not exactly "metal").
I don't know - maybe it's because death metal or heavy metal inspires feelings of violence and brutality that somehow makes a gamer feel "Tough" and "Manly", which is ludicrous, but there you go. Many people play games for similar feelings (myself included), so there's a link there. (Is Metal music Self-esteem boosting? Does it make you feel like a "bad-ass" (despite the fact that most gamers, like me, are shut-ins who can't take even one punch. You know it's true. I certainly do).