Most of the ones that are outright evil seem to be white supremacists. Eric Harris from Columbine was a true-blue psychopath, but otherwise there sure are a lot of shot-up synagogues and Black church prayer meetings and Latino area Wal-Marts. Histories of domestic abuse show up an awful lot, too.
And there's the mental health. Unfortunately, the way we find out that many of their mental health issues manifest violently is when they open fire. When they do go over that edge, they could use a bomb or a car or a claw hammer or something, but even the most die-hard NRA recruiter has to admit that nothing is as easy and effective as a gun. That's why they're around at all.
The US is filthy rich and full of smart people. I refuse to believe that if we stopped bitching back and forth about it and got down to business, we could come up with a system by which an 18-year-old walking danger sign who has been threatening a mass killing for years is denied a firearm, but the 99% of law-abiders don't get stripped of their rights.