From playing more than one GTA game a lot and also from learning about the effects it has had on people
There was a person who was deeply into playing GTA and one time he was arrested or something and he went and took a gun at the police station and just started walking and killing police officers dead.
If I ever had any children, I personally would not want them to ever play Grand Theft Auto. I am that type of person, but I accept that there are other types of people too.
A lot of things are legal and hard to criminalise but that does not mean they are ok. No one person, no perfect authority controls all laws and society. I am against too much censorship too, I guess.
I am not someone who would promote GTA. It is a bit like alcohol in Iceland where they have a state-run alcoholic drinks monopoly to discourage drinking. In that country, being seen with a bag from the state alcohol store (the only legal place to buy alcoholic drinks in Iceland) is a socially shameful thing. But of course, I am sure people there still drink. I admit that I did take this free GTA V deal but I feel bad about it.
I stopped drinking and never do drugs or smoke, though. Whoo!
Thats basically what behavioral psychology tried to prove about videogames for around 10 years, and couldnt.
Rough cut on why they couldnt is, that any effects that showed aggressiveness going up where temporary to under an hour (Basically people tilting, when you make them stop playing a game, without saving..
And 'in the moment' effects)).
And for anything else, like 'ultraviolence', driving over civilians, going on a rampage with Trevor (style gameplay moments), what it does in most people is actually escapism, and catharsis. So you move to this 'fantasy world' you act out your animal instinct behaviors for a while with no consequences. You notice and understand the difference between that and the real world. And after acting them out, they dont feature that prominent in your real life motivations anymore. (Curiosity, the forbidden thing, repressed instinct...)
SImilar thing with watching a boxing match. People knew about those needs to 'vent' in society (rules so more people can live closer together) a long time ago, and it is recognized, pretty much throughout society.
There is no 1:1 connection of playing a violent videogame, and becoming violent yourself. There isn't even a 1:100000 connection of that happening, basically.
People that are telling you not to look at the less socially accepted sides of humanity, because "it is bad" usually just are repressed and translate that into wanting to prevent something happening to you that usually doesnt.
That said, a 'normal' frame of reference (what is reality, what is moral, what is bad, ...) is needed to make those calls. Usually that is what friends and family provide during your upbringing. Not introducing PG18 stuff to children too early, serves largely the function of making sure they got imprinted with how society works first, before introducing them to undesired, sketchier, 'rolemodels'. That and if they see something thats very disturbing to them and they dont talk, and cant deal with it, because they are young, thats usually also an outcome you dont want to have.
But it should never be as simple as "played videogame > became violent and reenacted it in life", thats usually not how humans work. (Chance of less than one in a million, if you are talking about the example you used.)
edit: Here is another persons story, that basically tells you as much. Watched the video today, by chance, looking for GTA V retrospectives on yt.
(Btw, that persons opinion about the qualities of GTA Vs storytelling is wrong.
(That game is deep in its exploration of the shallow and deeper sides of humanity, that game is biting in its social commentary... Most of that went over the person who made the video's head, also 'making a point to run over every civilian on a bike, because thats what your philosophy teacher taught you' is not what he tried to teach you (callback to a line in that video), but hey... Who am I to judge.
(*judging*
))
edit: Desensitizing effects (f.e.) towards violence are provable though, afair. (Which is also why the military uses video games in training f.e.). Desensitizing means, you get less appalled/outraged/emotionally activated, when you see violent or abhorrent behavior, if you've experienced something like this (even in a 'fantasy world') many times before.