Gun massacres and mental health (the ones I know of off the top of my head).
Virginnia Tech, Seung Hui Cho - social anxiety, selective mutism, lack of emotional activity, was seeing a psychiatrist
Aurora, James Holmes - reports he was getting some sort of psychiatric treatment.
Hungerford (UK), Michael Ryan - suspected schizophrenia and psychotic behavouir, never diagnosed.
Dunblane (UK), Thomas Hamilton - nothing, anger issues
Utoya (Norway), Anders Behring Breivik - a big debate but at court it was accepted he had multiple mental health issues including narcissistic personality disorder, aspergers and possible paranoid psychosis.
Columbine, Eric Harris - suggestion a psychopath, but not diagnosed. Media left behind suggest mental health issues
Columbine, Dylan Klebold - suggestion a depressive but not diagnosed. Media left behind suggest mental health issues
West Side Middle School, Mitchell Johnson - nothing
West Side Middle School, Andrew Golden - nothing
Jokela (Finland), Pekka-Eric Auvinen - had used anti-depressants, media left behind suggest mental health issues.
Kauhajoki (Finland), Matti Juhani Saari - media left behind suggest mental health issues.
Cumbria (UK), Derrick Bird - nothing, anger issues
Tucson, Jared Lee Loughner - schizophrenia
All info taken off Wikipedia as to any diagnosis of a mental health condition. Arguable you cannot be sane if you go out to kill as many people as you can. So even the ones with nothing clearly mental had anger issues. Only the two boys at West Side Middle School appeared to have noting at all.
What is scary is most of the above got their guns legally.