“I’m going with prescription Psychotropic drugs again.”
No drug has the side effect of turning someone into a mass murderer.
I’m going with he was a thug, an evil piece of trash.
Most likely also true. But it has been recognized that the drugs can be a serious factor...
Just a little reading if you happen to find the time.
https://duckduckgo.com/?q=Psychotropic+drugs+cause+violence&t=lm&atb=v113-5_f&ia=web