Violent games DO give kids a different perspective on what is fun, fair and 'normal'. It is hard to believe that blasting away dozens of 'pretend' enemies doesn't nudge the consciousness of someone who already has issues with adjustment to society.
Of course, it is VERY hard to assert any direct, causal link..."after all, people have free will, and ought to know you just can't do that stuff in real life!".... right...

Yes, 'Dungeons and Dragons' is a lot calmer in general... my son played it a lot, but his group never even tried to inject violent aspects into it.

My view is that the creators of those 'games' have a lot to answer for. I really doubt that anyone ever called or wrote the early video game designers and said: "Hey, why don't you make a game where people get shot and chopped up with lots of blood & body parts!"...same with movie/TV program writers. They learned that a 'little' mayhem soon gets bland, and each tried to offer more mayhem in order to compete.... much like nudity grew gradually from those fairly tame early Playboys. There's no easy way to 'draw a line' on any form of entertainment, just as there's no easy way to pin down when guns crossed the line... but it's easy to see that they ARE way over the line of sanity now!