These kids, and any kid who commits violence, does so with full knowledge they are going to cause some trouble. Hell they wouldn't be doing it otherwise, because their motivation is the twisted pleasure of violence and control. When they get caught they realize “holy sheet” it's time to pay the piper and now start blaming it on anyone else who could possibly take the guilt and repercussion off of them. They are evil little kids who knew exactly what they were doing.
Now if you DO think that they should have all responsibility washed off of them, then I could place some blame on the parents since the game is rated Mature for strong violence and blood. If it's possible to blame someone else I vote mom and dad since they let their kids play the game. And even though these kids commit atrocities, mom and dad are there to nullify what they did and re-enforce evil behaviors as being OK!
Right and wrong, evil and good does not boil down to a video game. Or come to think of it, a book or a movie or a television show or a song lyric. Especially when several times in the past history of this country legal shenanigans of this nature have been tried as a means for guilty parties to escape reality.
I know allot of you agree with me, just posting my own take.