On the bus the other day, I was sat in front of an elderly couple, and overheard them talking about the stabbing of a teacher in school.
One of them mentioned that they'd heard "the killer played violent video games", with the implication that this was an important factor.
There are two things wrong with this.
1. So many people play "violent"* video games that it's completely unremarkable these days. It's like mentioning that the killer drank tea, or drove a car. Sure, it's true, but banning tea or cars wouldn't change anything.
2. Sample selection - you only hear about the notorious people who played violent video games. The vast majority who play them never cause any problems. This leads to an incorrect perception that video games are linked to violent behaviour.
Correlation is not causation.
It's just another media-driven moral panic.
* I put it in quotes because can it really be called violence when you're pressing buttons on a controller to make some pixels on a screen change?