I do think it de-sensitize people to violence to a certain extent.
I remember playing GTA with my siblings and it's obviously a game that's built to get the worst out of you and surprise you with how much one can enjoy doing things that are completely taboo and they wouldn't dream of doing in RL, you find yourself running civilians over with a stolen vehicule, following women and slaughtering them, killing prostitutes for their money, killing comes with riffles and throwing bombs at random cars because you can and nobody playing with you will be outraged at it because that's the point of the game. It's a vile game if you look at it with objective eyes, yet it's one of the most sold games amongst pre-teens/teens and adults, so if you enjoy the thrill of killing innocent people in the game and are slowly instable/have disdain for people due to being an outcast in real life or whatever, I would say that video games of this type could very much push you over the edge and train you to enjoy killing/seeing strangers suffer or teach you to enjoy the suffering one can inflict on others without guilt.
There is a youtube video of "elders watch" I believe, of people in their late 50s or more playing the game for the first time and most of them being initially outraged by the thing they are asked to do and then thoroughly enjoying killing people and/or slaughtering innocent passerby as the game goes on some becoming incredibly violent as time goes by.
It's to say that video games can wake that sadistic thing we all have in us, so while I don't think it's the main reason for shootings, I do think it's a big reasons as to the increase of violence amongst teenagers (be it between parents and children or teenagers and authority/other people).
I have worked as a nanny/teacher and I would lie if I didn't say the tantrums related to video games are usually much bigger and violent than the ones related to other things in my experience.