You might be interested in this excellent answer by u/veryshanetoday from earlier this year.

To quote their tl;dr:

All crime rates spiked from the late 70s through the early 90s. Serial killings were no exception. They were, and are, exceedingly rare, despite receiving excessive media attention. The answer to the question of why serial killings spiked in the 70s-90s is no more readily available than the answer to the question of why crime spiked in the 70s-90s, unfortunately.

Maybe more psychology-informed than historical, but several psychologists who specialize in psychopathic behavior have theorized that the same traits/personality profile that were common to serial killers overlap heavily with modern day mass shooters Same mind, different tool.

Psychopathy being a collection of personality traits, is much different from other forms of mental illness, especially not in the ways we currently talk about it. There are no treatment options, through talk therapy or medication, to alter a person's personality. The best mitigation is to limit access to tools, or identify early warning signals of psychopathic behavior.