Mental illness is an easy generalized cop-out. It is basic society. Contstructs life goals for kids at each stage of their life and ostracizes or critiques those who don't fit those plans. These are the clothes you will wear, these are the people you will associate with, these will be the classes you will take, these are the grades you will get, these are the extra curricular so you will do, these are the people you will take a sexual interest in, etc, etc.
I knew a guy in my high school grade, James, that wanted to execute multiple people in the school - preppy, obnoxious people who thought they were better than everyone else and made it known, teachers who accepted such behaviour, etc. Mentioned a few times to me about bringing a gun to school, but said he would give me a 'heads up' as I was one of the 'good guys' . Wiping the filth and sin from the school was his plan. He was very religious. Carried the Bible around with him all the time and was very quick to quote scripture to students and teachers alike. Very nice guy, and we got along well, liked to debate ideologies. He actually went on to become an ordained minister. But he would get so disgusted with people and their sin that I thought for sure he was going to 'clean house' one day.
"Disconnected youth" they call them. Blame it on mental illness. Not always - I don't buy it.