Offenders don’t “get away with” an insanity defense in New York. They might in fiction, but not in real life.http://www.nytimes.com/2014/05/03/nyregion/david-tarloff-is-given-life-sentence-for-08-killing-of-psychologist.html?_r=0
Tarloff was sentenced to life in prison. It’s likely Gilbert will also be sentenced to life in prison.
However, my point remains that serious mental illness can emerge during the late teens/early twenties, especially in young men. Drug use can cause or trigger mental illness, however it can emerge without drug use. Search for the term “psychotic break” on the Parents Forum of CC; you will pull up 41 pages of posts. http://talk.collegeconfidential.com/search?Page=p2&cat=15&adv=1&search=psychotic+break