Research reveals that people don't just "snap" or go off the rails. People who end up doing horrific acts leave a trail of red flags and clues. The kid who killed his classmates last week is a prime example of how parents ignored and dismissed serious signs of a troubled mind.You are projecting. I'm all for solving the problem of why someone goes off the rails and kills randomly. It defies logic. I think the answer is there's real evil in this world and we try to fix it with pills and talk. How to stop it? Man has wrestled with that question for thosands of years.
In psychology the word "evil" isn't used. There are definitions and descriptions for antisocial minds. I asked another poster earlier why conservatives can't admit the USA has a gun problem, and that also applies to any person. We have a bad habit of not looking at signs that a person poses a threat to others. Liberty is important to the American ideal, but then so is social order and safety. If the USA had better access to mental health services, and there wasn't a stigma for it, we would surely have fewer problems of violence.