These two seem to go hand in hand. And what better form of music to set the mood for shooting terrorists, building an empire bent on world conquest, or hacking orcs with your axe? Yes metal and gaming are the perfect mix.
Most specifically I would listen to Metallica's album Master of Puppets while playing Call of Duty 4 online. Songs like Battery, and Disposable Heroes were what really got me in a killing mood though! No seriously, I would usually put Battery on repeat and I would actually play better. Maybe it was the urgency of the music but I was near unstoppable when it was blasting.
I also played World of Warcraft for a while and never used to listen to music while playing. But let me tell you listening to the game's soundtrack over and over can get quite boring so eventually I would put something on. Usually it would be metalcore back in the day as I was (and to a degree) a pretty big fan back then. It never really affected my gameplay and was more background music than anything.
So! In the future find the bands/songs that suit you and get you into the game and watch the blood flow!
Now Playing: Edge of Sanity - Purgatory Afterglow - Silent