No amount of playing violent video games is going to make a normal, empathetic kid, teen, nor adult a killer.
The flip side is: no amount of shielding sociopathic kids, teens, or adults from violent media is going to make them empathetic or less prone to violence.
When the mechanism for empathy is broken in a human being, all bets are off. And treating a sociopathic person is FAR from easy. There are some medications that can help, if the diagnosis and behavioral treatment are there as well. But as population increases, more and more broken people are going to be scattered throughout the population. This is just the reality of the situation.
The solution, as I see it, is emotional evaluations... It might sound new agy and weird, but imo it's analogous to what schools are already doing with academics. Math, science, writing, logic, history and emotional development should all be standard fare for all kids. It would be easy to find out who had problems pretty early on, and avoid most of these tragedies. Hell, even so called "normal" kids could benefit from this stuff. I had a horrible time fitting in as a child and I attribute most of that to poor emotional development and a lack of social skills.