I think that this is just too easy. If a child want to play a game, especially if it is forbidden for him, then he will play it, and not even most good parents are IT experts, who are able to control what their children consume.
I'd like to know where the kids are getting fake ID to buy the physical copy of the game, or credit-cards to buy the digital PC copy.
n.b. I was talking specificly about my country where it is marked as an adults only product.
If you want to argue that we should censor all adult-targetted products because parents are unable to parent in this day and age, then go ahead....
Or must a game be violent to be fun ?
No, games don't have to be violent to be fun... But some are violent and fun.
Video games are a medium that's still in it's infancy -- we're about where film was in 1920, and still inventing what a video game is. Sticking to shooter tropes is easy, but there's many many great non-violent video games struggling to be seen amongst the gore.
Stepping back from video-games, to games in general --
Poker is non-violent, but chess is a war game where foot-soldiers' lives are near worthless when it comes to protecting the monarch...
Baseball is non-violent (unless you have a malicious pitcher), but Gridiron or Rugby are extremely violent, with people occasionally killing each other in hand-to-hand combat, despite their protective equipment.
Is it unethical to play Chess or watch the Superbowl?
The fact is that many people do happen to enjoy safe, recreational violence. That's part of humanity. Uncontrolled violence is a horrible thing, but safe outlets for it seem to be important in almost every civilization.
There's endless industries that cater to the enjoyment of safe or pretend violence, including but not limited to video games.
Personally, I think that trying to portray all of these forms of entertainment as unethical is a bit extreme... and it's not up to me to prove a negative.
Some people like getting into a boxing ring, some people like watching a dozen athletes throw each other into the dirt in pursuit of an egg, some people like pretending to shoot hordes of nazi zombies, some people like to play cops and robbers.
If you want to argue that catering to these people is unethical, go for it.