it's a complex topic with no easy answer. what has changed is the increasing isolation-yet-not-isolation that many school shooters seem to share, as many appear to be partially medicated, are socially isolated, yet have echo chambers and outlets for aggression online among like-minded fellows. that sort of isolation doesn't force someone to adapt and overcome adversity or change their behavior to something more socially acceptable, it seems to promote extremism and "stewing" where their impulses are rewarded with attention and a positive response.
Please excuse the snipping for length.
There is a problem, which is not confined to the US, with there being a growing class of people who don't fit in - or at least perceive that they don't and are therefore unwilling to try - to our society as presently organised.
There have been significant socio economic changes in first world countries in the past four decades.
Unskilled or lightly skilled manual and manufacturing work has been significantly outsourced to developing nations. In the sixties, even the class muppets were going to get some kind of job after school, and in most first world countries, employment protections and unions were stronger, so as long as the muppets showed up most of the time, they would stay employed. This is no longer the case. Work instability is common even in the professions now. At the sharp end, zero hours contracts and termination without notice are a thing. There are still unskilled jobs available, but they are in the services sector. Food, retail, etc. That means customer facing. That means the people skills to not flake out on a bitch customer and to turn up mostly washed. Muppets don't always have these. Not having these means a spotty chain of minimum wage McJobs as a work history. That means struggling to house yourself, let alone improve your standard of living with time.
Educational systems reform has meant that attainment is now measured to a greater degree by ability to turn work in consistently, and to do group work. Increasing importance of service industries has ensured schools have been 'encouraged' to focus on softer skills like working in groups and presenting. Some people are naturally bad at these. Some people have difficulties with the consistent motivation to keep plodding along turning in your papers. These people are now doing worse on the current attainment measurements. That makes them less likely to obtain employment or enter further education.
Learning disabilities are better screened for and supported now. But they are often supported in 'additional learning streams' within mainstream school. So now there is a clearly identifiable class of people who would have been the 'slow' folk in a mainstream school decades ago (when the only alternative was special school and they weren't THAT fucking special) who are marked out as retards but expected to share spaces with the normies who they perceive (and to be fair, often do) look down their noses at them for being speds.
Socio economic changes, often deliberately encouraged by first world governments to improve tax takes and keep housing prices high, have quickly reshaped the normie family model into one where both parents work. Normie mothers of small kids no longer vacate the job market, freeing up room for younger misfits and speds. Wages in comparable terms have fallen in terms of what lifestyle they can buy you. In the sixties, seventies and even most of the eighties, a standard desk job for dad would support mum at home and a kid or two without significant financial hardship. This is no longer the case. This not only keeps women in the job market, it makes a spouse who can't work or won't work consistently a huge drag on the family's standard of living. It very markedly increases the chances of the kids growing up in poverty, or comparative poverty for their area. This has the knock on effect of making a potential life partner who can't bring home a regular wage unattractive as fuck to young normies who can and do bring home a regular wage.
This leaves a small, but identifiable subsection of the populace who are disproportionately likely to be worse off now in terms of their life chances than they would have been if born forty years ago.
They are disproportiately likely to be young, male, low income, have an identified learning disability, have poor educational attainment, limited or no work history, a restricted IRL social circle, and to be single and childless.
There is a unifying factor over and above that that they share. They feel life has given them a rough deal based on the above, they are pissed about it, and they are able thanks to the Wonders of the Internet to seek out amd find communities of others like themselves who will provide validation, a sense of belonging and an echo chamber for their grievances when their IRL circles tell them to lose weight, get a job, put down the vidya, get a girlfriend, and they don't believe those goals are attainable for them.
The internet is a place where mostly anonymous strangers can egg other mostly anonymous strangers on in their violent revenge fantasies. Most of the time it comes to nothing. But there are some real idiots and some real unhinged motherfuckers in these echo chambers, too.
Sometimes they get weapons, and they actually fucking do it.
This isn't a problem that will go away in first world countries without a lot of serious thought. In the UK, our gun laws are strict as fuck, and our speds go join Isis. In the US, your speds take guns to school.
You can pass laws making it harder for speds to do violent sped things, but reducing the number of dropout speds is going to be a lot fucking harder, and it's a problem that the political class doesn't want to look too closely at.