We have lived in much more violent times than the present.
There have always been stories of battles, wars, murders ect but with time it has evolved into a world we can be thrown into, but just because we choose to play violent games, in my opinion, does not make us more likely to act these out.
If I had to categorise people into some kind of groups I would imagine it would be like this:
Ineffective upbringing - not instilling the correct values, or teach the skills of reasoning / right and wrong.
Impossible upbringing - where there is some medical background to as why the individual can not reason correctly.
Environmental factors - if some one breaks into my home and I feel my family is at risk, be rest assured I will allow my standard reasoning to be overcome for their safety. The same situation I would apply to a victim of a mugger or armed forces or indeed people born into a world of violence.
The "snap" - where something triggers a sudden loss of our standard reasoning.
Finally, the plain ol crazy - but to be honest I think they would have just got there the same without videogames.
So yeah, in conclusion I place little to no weight on the video game argument.
*disclaimer - coffee shop ideas only 😛