If you look at the mass murder rate before and after the mental hospitals were closed you will see murders took off like a rocket after closure in the 1970s.
Before that time the crazies were locked up. They try to blame it on firearms but such firearms were around since 1903. Yes, 1903! And you could buy them through the mails, cash, no background checks, no paperwork. And they were available everywhere! Gun stores, hardware stores, book and record stores, hat shops, gas stations, saddle shops, clothing stores, liquor stores, grocery stores. You could even buy an anti-tank rifle through the mails.
Here is a Remington Model 8 designed in 1906. Yes, 1906. This one was made in 1935.
Notice the uptake in mass murders in 1970s when the crazies were let out of the mental institutions,
https://en.wikipedia.org/wiki/List_of_mass_shootings_in_the_United_States_(1900%E2%80%931999)
Even when Mayor Koch tried that mental health van to do street evaluations of the craziest homeless for possible commitment, the operation was quickly shutdown by lawyers and activists.
N.Y. Picks Up, Commits Its Mentally Ill Street People
Los Angeles Times
36 years ago — They undergo psychiatric evaluation, and can be held for 48 hours involuntarily, if two psychiatrists agree. Patients who are admitted are ...
https://www.latimes.com/archives/la-xpm-1987-10-30-mn-11718-story.html