(I mean to say that it is A factor, not THE singular cause)
Or rather it resembles a mythical beast, part of the hydra of American societal collapse. We're all alienated and isolated from each other, further and further removed from viewing each other as fellow human beings or fellow citizens in a nation. What causes someone to snap and arbitrarily decide to find a crowd of people (frequently schoolchildren) with the express purpose of committing mass murder? We know that entitlement and rage play huge parts, as does this alienation. If you view other people more as competition, as enemies, as obstacles or threats, you'll more likely find it easier to commit mass slaughter.
Capitalism splits us apart like this. As does car culture. Americans are so fucking car-brained, another cause/effect of this alienation. Waiting in a queue, already intolerable in a standing-and-waiting context, infuriates even the most patient person if they're "stuck in traffic". Fellow citizens become threatening obstacles by default. When driving, the only real means of communication are expressions of rage (horns, screaming, middle finger, et. all). You're not just waiting to place your order in a cheap restaurant or coffee shop, you're stuck in traffic that's backed up onto the street waiting for the drive-thru.
When driving, pedestrians are inconveniences to your personal space and entitlement. Other cars are even worse; barely recognizable as human-operated, they're metallic monsters rushing at lethal speeds around you, cutting you off, getting in the fucking way, going to slowly or too fast, and making your day that much worse. Bicyclists are demoted to literally subhuman in the driver's mind.
American society is rapidly innovating new and exciting ways to reduce face-to-face interactions between humans. Kids are packed off to school, leaving the adults to work remotely or in jobs whose main role is getting screamed at from a car window. We're preferring more and more to remain in our pod-homes, interfacing with our friends and coworkers and bosses and the world through a computer screen. It's easier than ever to shop for anything from the computer too, further reducing time spent outside with its distasteful human interaction.
When it's time to leave the home-pod, our default behavior is to seek the comfort of the familiar and travel in the mini pod. This pod will rocket around cities at high speeds, coming uncomfortably close to other such traveling pods which are concealing their own fetal occupants. These metallic wombs grow ever larger, more spacious, more luxurious and even decadent, at the expense of all life and environments around them. They may park at a store where an underpaid clerk will rush out goods to be loaded like ants shuttle crumbs to the nest, the drive-thru experience brought to groceries with equally minimal human interaction. Or perhaps they'll crawl through a fast-food line, delivering automatic orders on apps or chatbots and waiting for a hand to thrust a bag into a side orifice to deliver nutrients straight into the human's gullet. Maybe one car will smash into another, at which point other cars will arrive to deploy their own special symbiotic humans who may deploy even more depraved violence of their own.
Like a triple-headed ouroboros, the alienation and violence and cars all feed and birth each other.
A lot of this is also applicable to suburbia and I have also personally suspected that most of the right wing mass shooters come from "middle class" suburban homes. They feed into each other to practically guarantee a high level of atomization and loneliness in most of the people living there in my experience.
To expand on this, I remember seeing the claim that the FBI found that the serial killer phenomenon's rise correlated with the rise of suburbia, saw that either on here or the trueanon sub. No idea on the validity of that but it makes sense to me, also synergizes well with the idea that mass shootings have replaced serial killers in a way. The brain-breaking alienation and atomization was always there, the extreme manifestations of it just switched to a much shorter and more violent burst now for whatever reason.
Suburbia, car culture, and the nuclear family are the perfect methods of passive misery. For anyone trapped in it it's practically inescapable and like a fish with water it'll be the last thing you notice if you've never experienced anything else, especially for children.
The Mass Psychology of Fascism by Wilhelm Reich.jpg
Adds to "want to read" shelf in goodreads
If either the home-pod or mobile pod are under threat, the occupant is allowed and encouraged to kill the threat.
When you live in suburbia you can effectively tune out all other people and live in a bubble, with the internet being your only connection to the outside world, along with grocery trips and work. That's harder to do in a city.