People who claim mass shooters are automatically mentally ill are trying to tar shooters with the very stigma so many have tried so hard to eradicate. They call them mentally ill in an attempt to degrade them. That's an insult not to mass shooters, it's an insult to the mentally ill. You want to degrade these shooters? Call out what's really wrong with them.
Misanthropy isn't a mental illness. There are lonely, embittered and insignificant men, furious at their own insignificance and with a rage to take it out on innocent people, and then die. They have hatred and contempt for the woo woo hallmark happiness of others, sold and marketed to everyone, which to them is nothing but a lie. They want to prove how meaningless life is, by confronting the world with senseless death.
What are we supposed to do, create a world in which everyone can make meaningful interactions and find purpose in service to others?