While games are again and again pushed to the spotlight for such things it has nothing to do with games themselves.
Politicians and other groups look for someone/something to blame when bad things happen and well, a few decades earlier it was Rock'n'Roll and before something else and now its them video games..
The real issues turning kids to mass murderer maniacs are others, they are usually severe problems in the kid's social life combined with bad outlook for the future feeling and various other problems. Often many of those are directly or indirectly caused by the school system, financial/work system and hence educators, politicians etc (Next to that, yes, that it is very easy to obtain weapons in the US sure does not help when a kid with such severe issues can easily get a gun or similar) Guess which groups would love to blame games most to push the focus of blame away from themselves?
One can create as many studies as one wants, waste of time, one would instead just have to look at all the people playing video games to easily see that there's only a tiny fraction of people among all those who are uncommonly aggressive etc, not a larger group than in any other group of such total size. But yeah, no,... its them video games..