Probably a bit of both and lack of parental supervision and care.
I’m 24. I grew up on violent everything, including video games. I even own the Soldier of Fortune series, undoubtedly the most gory shoot-em-up ever made. Never contemplated anything of the sort.
And if you play - you see millions of others playing online as well. There are always screwballs, but this recent phenomenon is coupled with parents not raising children properly.
Blaming this on videogames is like blaming Columbine on guns.
Parental responsibility.