While this is true, please remember that research like this is age dependent what is good for a relatively advlt person will not always be equally good for a younger one. The younger the child is the less distinction is made by him between gamin behavior and behavior outside the gems - when we are children games are not outlet, they are a way to learn. For an advlt person it is not so, and letting off steam through violent games have a positive effect.
The age at which violent games became more of entertainment and less of an important part of life that lets you learn how to react to your environment is different for all people, and it takes someone who actually knows the child or young man to tell if it's ok for them to play certain kind of games. That is why parents decide what games they should buy.