There is much speculation on this subject. As a parent I have experienced the horror of my child texting and saying, ". . . the school is on lockdown, I've been on the floor of the library for an hour, I'm scared . . ." This is one of the worst things you can hear from your child when they are supposed to be safe at school.
Are all the killings in schools committed by mentally ill people? Not necessarily, although mental illness can play a major role. Christopher J. Ferguson, associate professor and department chair of psychology at Stetson University said in an article after the Sandy Hook shootings, "Our country's funding for mental-health services has only gotten worse since the 2008 recession. As the National Alliance on Mental Illness has been warning for some time, the existing level of funding is inadequate, so our nation's ability to identify and care for the severely mentally ill has been hamstrung. . . Obviously, the vast majority of the chronically mentally ill won't commit crimes, certainly not the severity of the Sandy Hook shooting. But by leaving the mentally ill adrift to fend for themselves, we miss the opportunity to identify and treat some of these at-risk individuals before they escalate."
According to Phil Chalmers author of Inside the mind of a Teen Killer, ". . . kids give off warnings signs and there are things you could look for. He says bullying and unstable homes are top reasons why teens commit the crime. The teen may also display suicidal feelings or become obsessed with violent media."
Dr. Peter Langman presents the psychological causes of school shootings and offers insight into why certain teens exhibit the potential to kill. He shows how to identify early signs of possible violence and offers preventative measures that parents and educators can take to protect their communities. http://www.schoolshooters.info/PL/Prevention.html
A young person's brain is not fully developed until they are about twenty-five. Some of the fundamental stepping stones to maturity, such as emotions, impulse restraint, and appropriate social behavior aren't fully functional. Our teens need guidance and validation. They often suffer from the inability to make rational decisions and the lack of a qualified support system. Be there for your teen and support them, don't leave them adrift to fend for themselves.