The anti-bullying efforts that have picked up steam in schools over the past several years? Those came about in part due to research into school shooters, and awareness that bullying and social alienation play a role in some such crimes (in addition to increased awareness how much such behavior contributes to teen suicides).
It should be recognized that the vast majority of people with mental disorders - any sort of mental disorder - are not violent. Statistically, people with schizophrenia are vastly more likely to harm themselves or to be victimized by others than they are to harm anyone else. Statistically, college-age males are more likely to engage in violent behavior than are mentally ill individuals. In fact, the single best diagnostic predictor of violence is substance use. And yes, that is use, not dependence. Substance dependence adds significantly to the risk of various sorts of other criminal behaviors, but not to risk of violence per se. The binge-drinking 20-something is every bit as likely to be violent as the chronic alcoholic or meth addict, and vastly more so than the average schizophrenic individual.
The guy behind this latest killing, for example, apparently had Asperger's disorder, which is a type of relatively high-functioning autism, in which the individual is socially awkward and more or less clueless about social cues. As a group, Asperger's individuals don't tend to be violent at all.
Many people who are disturbed are lacking in treatment, though. There are many reasons for that, but they largely boil down to financial. Insurance companies don't want to pay for effective medications, which are often expensive (some of the newer antipsychotic and antidepressive meds can run hundreds of dollars a month for a single medication). Many people don't have insurance at all, or have prohibitive copays. The meds themselves often impose unpleasant side effects. Ask yourself this: How excited would you be about taking a daily medication that makes you groggy, causes you to gain weight, increases your risk of diabetes and causes dry mouth, nausea and sexual dysfunction? Choosing between that and one's symptoms is the sort of choice faced by many mentally ill individuals on a daily basis.
There is also a serious lack of mental health expertise in the US. Relatively few physicians choose to go into psychiatry, mostly because it pays poorly as medical specializations go and carries a lot of costs and risks in terms of malpractice and liability insurance(something unavoidable when you are dealing with a population with high risk of suicide). Some surveys suggest that nationwide, we need something like 33% to 50% more psychiatrists than currenty practice in the US just to meet current demand. The same is true of other mental health disciplines. 50% of the clinical psychologists, clinical social workers and licensed mental health counselors currently practicing in the US are projected to retire within the next 15 years, and fewer people are entering those fields than in the past - again, largely due to financial factors. When you compare the amount and cost of schooling necessary to practice to the wages earned, mental health fares very poorly compared to any number of other disciplines (though philosophers, art historians and archaeologists have it even worse). Why go to 6 or more years of college + internship in order to make, on average, less than 1/2 what someone with a MBA makes?
The shortage is particularly bad when it comes to people trained to work with children, which is particularly tragic, because a lot of research shows that having trained providers available in school, and teaching kids some very basic psychological resilience skills at a young age, helps reduce incidence and severity of any number of mental health problems in kids and adolescents.