Im going to sound like a know itall here so i apologize in advance. But the first thing i learned in my schooling around abmormal psych is that people never, ever, just snap - at least not in the way you describe as if a switch just went off. There is an underlying stressor or an unreported or ignored mental illness. In news reports you'll hear all these people tell how normal a murder suspect was, but this is mostly because they either put on a great facade and manipulated everyone around them, or the signs that were everywhere were missed or ignored (like the high school shooters in Columbine).

It is the same physically. Take a healthy young person who has a massive heart attack during a basketball game. Their healthy heart didnt just turn on them, that's just how it looks. When they dig doctors find out there was some congenital abnormality no one ever knew about. Or maybe the kid was prone to taking massive amounts of stimulants along with a couple of energy drinks before a basketball game. The public usually wont ever hear about this. The report is that the kid jut dropped dead from a sudden heart attack for no reason. But really, that just doesn't happen.