http://gamerant.com/video-games-increased-aggression-violence-study/
Aren't there like...17 more studies that say the same thing?
On one hand, almost every game released these days have multiplayer, and all these multiplayers have it that the only way to interact with another human player is to shoot, knife, destroy them, and if you dare try to ask any of these online people for assistance, you're replied with racism, sexism, and curse words. Being a bad parent and exposing your easy to influence 9-yr-old to this environment and make him think it's the norm to act aggressively to other humans could in fact cause him/her to grow into a more aggresive than necessary type of person...
On the other hand... Parents, control your children.
There are also studies that say games make people less violent. And studies that say there's no effect at all.
What is fact is that over the years, as video game sales have increased, violent crime rates have dropped. Not saying that the two are connected, but that is something that is happening.