Personally..

I think people overreact to isolated cases of killings which appear to be linked to violent video games. These murderers clearly had something wrong with them to begin with; and would have found inspiration elsewhere for murders if not in videogames. The fact that the victim was playing the game in the manhunt case, as opposed to the murderer, is proof of how reactionary the media can be; without investigating properly, the issue of violent video games is certain to attract the attention of outraged parents.
In the case of these parents: it seems any excuse is sufficient for them to point the finger of blame elsewhere, rather than look at how they themselves bring up their children. It's quite hypocritical to complain about violent games being played by their children when they're the ones who buy the games for them, despite the big '18' certificate on such games. And regardless, any child will say they find the killing fun, just as how any child will at the same time point out it ins't the same as killing in real life. It's pretend killing, no different to playing cowboys and indians. And even if a game provides inspiration for one or two real life killings by some deranged psychopaths, is that justification for denying millions of sane people that game? And is it justification to attack the whole media of video games? I don't know about you, but I don't think so.
That said, I do agree that there is a limit to explicit content in a game. There is a difference between violence in a game to go alongside the game's theme, and having violence itself as the whole theme of the game. Nevertheless, even if the level of violence is grossly distasteful, I do not believe that necessarily influences people to kill. I'd say it's more inclination to switch the game off; relentless violence with no break would become tedious.
Then again I can't claim my opinions as absolute; everyone reacts differently to things.. and I'm not exactly a fan of most shooting games. They're fun for a little while, but shooting things quickly loses its appeal for me, so perhaps gamers who get more immersed in them are more likely to be influenced by them? Hmm, I'm not so sure, seeing as when I was younger (supposedly making me more impressionable) I enjoyed these games alot more, yet did not ever get the inclination to shoot someone in real life. But everyone's different, so thinking reasonably perhaps there should be a limit; nevertheless, the fact that everyone is different is all the more proof that parents should think carefully about how they bring up their own children, rather than simply finding a scapegoat in the form of video games.