My response to this issue is always the same. A parent should always have knowledge of what their kids are playing. Now, I don't mean harping on the kid night and day, but talking with their kids is best when it comes to violent videogames.
I've known one or two people who have had their minds skewed by violent video games, but I believe it's just a mistake of discretion.
To be honest though, kids nowadays are becoming increasingly more violent and numbed to it. My mom works at a school with 7-11 year old children. Boys there are already forming "gangs" and girls are already forming "posees" and due to this activity they are not allowed to have more than three people gathered at one time during recess. Something of that nature never happened while I was that age, and if it did...it was few and far between.
My point: Parents should use their own discretion on videogames. A violent videogame is never usually the only cause of violent real time outbursts.