Firstly, six billion and counting HUMANS being at peace? It'll never happen. The only time humanity could unite is if faced with something which threatens their existence: the Formic (Bugger) War, or something similar. And that wouldn't be peace at all.
But, let's assume that this was someday the case.
Violent games could still be made. The majority are either in fictional settings, or World War II which ended 65 years ago. When a game (Haze, Six Days in Fallujah) tries to be set in a real and current war, people become upset and the game is canned or drastically changed. Modern FPS are about fighting terrorists (which is fine, it's not a war), or occasionally something ridiculous like war with China. But I digress.
Now, if you added on "no crime" (which goes from extremely unlikely to downright IMPOSSIBLE), I can understand where you are coming from. A few generations on, people wouldn't know what violence was. They would find it grotesque, and yes, games on the whole would probably become a lot more violent, if only because everyone who could stand violence had presumably been shot to stop them committing a crime at some point (Minority Report, anyone? 
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