Mass Shooters aren't necessarily crazy just angry

So, in some ways it is not about mental health (at least from a clinical perspective). Often mass shooters are completely sane just very very angry.

There could be many many reasons for their anger but the most likely reason would be some kind of real or perceived abuse from the people they want to shoot if it is specific individuals they are shooting. Or it could be anger from perceived abuse from an ethnic group of them or someone they know during their lives. There are many reasons why people eventually become mass shooters. But, the biggest reason likely would be that the shooters basically don't understand themselves very well.

What I mean by this is that instead of dealing with their own problems they are projecting ALL their problems outward upon other people which is never very healthy as a response to fear and anger.

Anger usually starts as fear and then eventually moves towards anger. But, fear can be from many different sources like the boogieman some of us as children fear under our beds or in our closets as children. When that fear goes out of control in a person, this is when it turns to anger and rage and when this is acted upon it becomes a homicidal-suicidal rage that leaves many people dead often including the shooter.