I agree with everything you wrote above. But all these shooters previously listed had a mental disorder of some sort. This isn't some Gang Bangers planning and executing a shooting of their Rival enemies. These Loners were sick in the head period. That doesn't mean they couldn't methodically plan their shootings...
There's not a soul alive that couldn't fit a definition listed in the DSM 5.
It's a slick way to label people for disarmament. If it's in the book, you can't be trusted. It's one of the most powerful propaganda methods I've seen. It's absolute.
Your free will is not your own. It's chemicals in your brain. You're unsafe.