I sat and spent a fair amount of time on a piece of a 2015 study by Michael F. Stone, a PhD psychoanalyst from New York. He sat down and spent a fair amount of time looking at mass killings.
For his study, he ended picking 235 mass events (only from the US), from the period of 1913 to 2015. My guess is that he picked events which had a fair amount of written material, but it's unclear how the 235 were really picked.
One might also note that he didn't go out and interview witnesses but just relied upon the newspapers and books written.
The FBI will tell you that roughly one-thousand mass murders occurred between 1900 and 1999. Since they didn't come along until the 1930s.....I might go and question how they arrived at their number but I'm willing to accept some basis to start this discussion from.
In most of these 235 events....Stone comes to a clearly defined end-result....at least 228 of these were folks who had some type of mental illness, psychosis, schizophrenia, or bipolar.
Basically....well over 95-percent fit into his profile of people who weren't capable of grasping the event or what they were doing.
The Vegas shooter? Well....if you go by what the girlfriend said....he was showing signs of schizophrenia. The doctor giving him mood-altering drugs? Nothing in terms of a pubic statement but the fact that the prescription started about a hundred days prior to the event....would lead you to some view of the serious nature of his mental condition.
This kid in Florida? Based on various descriptions, I'd take a guess he was on medication and probably bi-polar.
The problem here? Let's say you opened up the books and admitted out of the past twenty years....98-percent of all mass murders were triggered by mental illness. What exactly are you willing to do? Would you be willing to confirm six-hundred people per week from across the US to a permanent and ever-lasting mental institute? I doubt it.
If they wanted to kill a mass group and didn't get a gun? Well....you'd go and explore various other ways from toxic gas, to poison.
The 9-11 crew? I think if you really dug into their mental side....most would be defined as having some form of mental illness.
We've got a problem but it's clearly not going to be solved by discussing gun-control. The study by Stone? I'd highly recommend it but it's pretty long.