Bullshit. Cain played way too much GTA, then went out and killed Abel. Duh. Fuck your study.
But seriously, every time a common sense study like this comes out, someone is quick to release another "study" and paste it all over news sites everywhere about how "video games are destroying innocent children".
It's pretty safe to say that I've played a LOT of very violent video games, yet there hasn't been a single time that I've ever hit somebody unprovoked, and even in those cases I felt DREADFULLY sorry about doing it. So if violent video games a murderer make, well, it's only a matter of time before I instantly switch from "wouldn't hurt a fly" to "shopping mall massacre".