A striking fact about the deadliest mass shootings is their geographic distribution, lying mostly in areas that are heavily colonized by transplants, rather than in places with deep historical roots and communities.
This tends to be a fractal phenomenon -- at the regional level, they're most likely in the Sun Belt, but even within such states, they're from newly founded suburban enclaves (ironically intended to be "safe" unlike the dangerous old towns and cities nearby), and often the killer's family and perhaps the killer himself are transplants (including immigrants or sons of immigrants).
They are unlikely to happen in deeply rooted places like upstate New York (unless it's an immigrant like the Binghamton shooter), or Ohio. If there's a spree shooter from northeastern Ohio or western Pennsylvania, he'll end up killing people in a transplant area like the San Ysidro McDonald's shooter. For that matter, cult leader and mass murderer Jim Jones was from Indiana but picked up the most followers once he moved out to San Francisco, and then really lost it when he moved them all down to Guyana.
Rootless places attract people guided by a laissez-faire approach to behavior, who don't mind throwing away the constraints of living in a place where traditions are strong, family ties are extensive, and even strangers know each other. Perhaps that's the exact reason they're fleeing a rooted place -- so they can just live their own lives without being bound to others.
As a place comes to be colonized more and more by such people, normlessness becomes the norm. Do whatever, say whatever, think whatever. Fuck the haters. If it feels good, do it. If you got it, flaunt it.
The residents may think this low-lying level of anti-sociality is no big deal. They're being true to their individual selves, and at worst it produces bratty and entitled children. Nothing devastating, certainly not worth going back to the rooted environment with all of its constraints on individual behavior.
But it's precisely this code of "do whatever" that allows the small handful of truly warped individuals to carry out their warped fantasies. Typically these are revenge fantasies (rape or murder), as the social reject killer has no other ties to sustain him, and seeks to lash out at those who have rejected him. He has no extended family, no neighbors, no other communal links that would keep him calmed down despite being a loner at school.
Once he stews in those revenge fantasies long enough, what constraints are there to hold him back from carrying them out? He lives in a world of his own, with no palpable policing presence.
Although such cases may be rare, their impact is catastrophic when they do hit -- there could be dozens of innocent people brutally murdered for no reason in a public spectacle. In addition to the immediate loss of life, there's the permanent scar left on the area. It is exactly these rare-yet-catastrophic cases that social norms are supposed to protect against. They may feel annoying sometimes when you want to do your own thing, but they're there for the greater good of preserving the community, like a form of insurance.
These kinds of spree murders have become more common as more people have dislocated themselves and their families in pursuit of higher career prospects. It's generally not dirt-poor people reluctantly moving to the nearest city after the good jobs vanished in rural areas. It's middle class people moving from Nowheresville to an up-and-coming "it" place. This is what makes the shootings so counter-intuitive to most observers -- they happen in middle-class enclaves with good schools and promising children.
The status-striving trend began with the Me Generation of the 1970s, and has only gotten worse since then. So has the concomitant moral code of laissez-faire, which most middle-class people probably assume is written into the Constitution or the Bible at this point.
Only when we reverse this trend by staying put where our roots are, and accepting the duties and constraints that this places on our behavior, will these kinds of warped revenge fantasies no longer be thought of, let alone acted upon. The moral code will change from "do whatever" to "rein it in for the greater good".
That will mean denying yourself the attempt to climb the status ladder by moving around all over the place -- but by now that's mostly a fool's game anyway, all of the good spots having been taken and held onto for awhile. If a handful of people do this, it may not wreck society, but if enough people uproot themselves, then the entire society gets destabilized -- behaviorally and morally.
To put it bluntly, we do not have the right to "do whatever" as long as it doesn't immediately harm others. Acting as though we did have that right leads to patterns of behavior that, after a sufficient percolating delay, cause far more destruction to ourselves and others than we imagined was possible.
There's a contradiction in the liberal approach to these problems, where they hold the role of the government to be regulating the society to be safe for libertinism -- to allow law-abiding individuals to indulge in as much atomized laissez-faire behavior as they feel like. Allowing AR-15s to be sold to teenagers conflicts with the safety of the do-whatever enclave, who may become targets and have to watch their back rather than go about their do-whatever-ism. So they want that practice regulated out of existence.
But you can't push for tough regulations on other people's behavior without accepting more regulations on your own personal behavior. Pointing to potential "harm" done by the other person is no good, since your own laissez-faire behavior is corroding and destroying others, just not in as concentrated of a way. It's long-term and diffuse, but no less offensive to social norms.
And of course the ultimate form of regulation comes from feeling social pressure, whether from extended family, neighbors, peers you've known your whole lives, and so on. Accepting tougher regulations on your own behavior means accepting the plan to stay put where your roots are, rather than wander toward anonymous crowd-places like some Gypsy thief.