Most mammals, humans especially included, use games to teach and exercise survival skills, including fighting, hunting, hiding, etc. So it's to be expected that many video games serve that purpose.
It might not be clear at first just what video game killing has to do with real killing, but factors like teamwork and delegation, prioritizing goals, managing space and time, and adapting to changing circumstances are all present in various forms when violence comes a-knockin'.
Of course, I don't mean to imply that the purpose of violent video games is to teach fighting skills. That's silly; as silly as to say that sports are deliberately intended as ritualized depictions of warfare, or that kids understand when they play Hide & Seek that they are learning how to evade invaders. Rather, they appeal to the same deep-seated instinct that drives all play-fighting. We like imaginary combat. It is just one of many things we like, but it has an edge over several of them because it's exciting and glamorous in a way that imaginary business or imaginary work are not. Furthermore, it's pretty easy to write rules to represent the concept (even before you get into physics being easier to simulate on a computer than feelings).
