ATTACK - 30 Seconds to Mars.
I like the song, but the reason I choose it, is because in the chorus they use the lyrics "Run Away!" which is just perfect for a killing spree.
Also it's name is fucking "ATTACK".
If not that song I'd have to pick:
Getting Away With Murder - Papa Roach.
Just listen to the freakin' song name. It's perfect.
If I had to pick a song, just because it all around fits, I'd probably take:
Ladies and Gentlemen - Saliva.
It's just plain awesome and gets your arenalline pumping.
Edit: Awww, you ment while playing video games? Bummer! Thought you ment a real slaughter IRL.
Then, when playing video games, I'd have to pick:
What's this? - Nightmare Before Christmas cover by Fallout Boy. It just makes me feel really happy which really increases my gaming performance.