I work with people with mental illnesses, as has been mentioned drug/alcohol abuse is a major factor, as is stress ...(many people I have dealt with have been uni students, got too stressed and its tipped them over the edge...a fine line between madness and intelligence eh)

Something happeneing in your lifetime can also contribute....deaths of parents/close friends etc...

There is no definitive answer as to why somebody may go that way, lots of varying contributing factors etc etc. There is some scary shit happens though and ive seen it first hand, its not pleasant

One of the worst scenarios I was involved in was at a residential home where this woman had been on a downer for months. I got involved as I was working as part of the Crisis Team at the time, got a call out on a night shift at about 2am. Got there to find this woman gouging the skin off her body and throwing it at the walls!!

She later died due to extreme blood loss etc