One can say that Hitler’s SS henchmen were completely sane too. They knew what they were doing in murdering innocent humans for their twisted greater cause. Good point.
You don’t have to be insane to be evil, of course.
If there is a common thread in a lot of these shootings it may be the frustration of a teenage/early 20s male with limited social skills who simply isn’t getting any, especially when you perceive that lots of other guys are.
I never contemplated going on a shooting spree, but when you’re that age it’s an unbelievably frustrating preoccupation.