Video games can cause agression if the game is violent, but they don't necesarily cause more violence.
For the sake of consistancy, I will assume the child playing is between the ages of 6-10
I think it's perfectly fine to allow a child to play a violent video game, but at a young age like such, certain games should be kept out of their reach.
While I feel its perfectly fine for a child to play soul calibre, the call of duty franchise (to an extent) Turok, etc. I feel that certain games should be reserved for an older audience because they have more mature (or in some cases less mature) instances that a child would be exposed to too early on, or that they wouldn't fully understand/comprehend because they are not well versed in the subject, and could cause problems if they mentioned them among peers. Examples including games like grand theft auto and duke nukem, which while fun games, should be reserved for a slightly older audience because they don't just deal with things like violence, but with matters of sex, and similar subjects that a child at that age should not worry about. (notice how I say not worry about instead of not see.)
EDIT: I understand there are parents who would say otherwise, and people who would claim they were exposed to sex/extreme gore/similar mature topics at a young age and turned out fine, but as a generalization, a child is young and inexperienced in these aspects, and while its perfectly fine for a parent to explain these things to a child, they would probably be better off not learning about these topics through games and such.
