Fort Lauderdale gunman was "receiving psychological treatment"

[spin]

2. The theory linking antidepressants to mass murders has been around for a while. ...

I don't know if any studies have been done to to verify or disprove this theory. I suspect the pharmaceutical industry would oppose such research.

You can find information about this theory by using Google. Many of the links will take you to conservative sites.

Antidepressants have been linked to suicide.

Suicide & Antidepressants

While antidepressants are designed to decrease the symptoms of depression, they occasionally have the opposite effect and can increase suicidal thoughts and actions.

Patients who take selective serotonin reuptake inhibitors (SSRIs) such as Prozac (fluoxetine), Paxil (paroxetine) or Zoloft (sertraline) may experience side effects such as violent behavior, mania or aggression, which can all lead to suicide.

What begins as withdrawing from friends and activities and a loss of interest in work can escalate to harming oneself. In clinical trials and public use, there have been cases where antidepressant users have thought about, attempted or committed suicide.
https://www.drugwatch.com/ssri/suicide/

I actually have know a person who was taking an antidepressant to help him sleep. Shortly after he started taking the drug he was driving on the freeway and suddenly had an almost overwhelming impulse to drive his vehicle into an overpass abutment. He managed to fight it off but it really scared him.