I think violent games actually have at least two peaceful effects.
One is they give an outlet for aggression and frustration that has no impact on the real world. Games can also be about violence in a way that satisfies other tastes for violence, and sometimes that’s a fascination with the reality of tactical situations, historical situations, and so on.
Another is when they are done realistically, they can actually give players the experience of violence in a negative way. Most games aren’t like this, but some are, and it only takes a bit of experience with the awfulness of violence to show why one would not want to be violent in real life. That is, games that let you know characters as humans with personalities and stories, and then when and if you involve them in violence, there’s a good chance they’ll get killed or maimed and there’s not much you can do to ensure that won’t happen, it can make a big impression.