It's not nonsense as such. The evidence is perhaps not unquestionably overwhelming, but it does exist: violent video games do contribute to increased violence.

It’s just that there are some caveats:

Those most likely to engage in subsequent violence are the ones who are the least good at them; they get frustrated and angry, cannot sublimate that anger (because they cannot progress any further) and, therefore, take that anger with them into their external life. If they hadn’t played a game to begin with, they would not have been frustrated and would not, therefore, have been angry — it’s a bit of a circular argument, insofar as there’s no way to know that wouldn’t have been angered by something else but the fact remains that it’s a source of frustration that didn’t previously exist.

Secondly, they are training grounds for successful violence.

Previously, some whackjob might pick up a gun and take a few potshots at people but they weren’t a trained markswo/man, mostly missed their targets and, on the odd occasion when they did hit someone, had been aiming for the torso and their victims mostly survived. And if their victims were police then even when wounded they were in a position to put an end to things thanks to being armed and trained.

Years to decades of being trained by videogames to go for headshots has resulted in people who are not simply better shots (used to successfully hitting that very small target) but, furthermore, more fatalities: a headshot is pretty much lethal every time and, armed and trained or not, that means a whole lot less resistance to a spree killer and they can, therefore, go on to kill more than they used to.

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So, it’s not that they cause more violence necessarily but what violence they do result in … and any other violence caused by those with experience playing such games … is all the worse for it.

Their impact is, therefore, not insignificant.