Mustang, I think you're onto something talking about losers lashing out against "them" in your #94 post. I think a lot of these shooters are basically lonely guys who can't stand watching people succeed while they can't get laid, can't get the right job, or otherwise can't achieve their goals.
We've all met people like this. They have a chip on their shoulder and obsess over perceived slights. They feel entitled to greater happiness and think it's those people over there (blacks, whites, Christians, Muslims, women, jocks, sorority girls, foreigners, poor people, beautiful people, etc.) who are preventing it.
I think a lot of these mass murderers feel disconnected from mainstream life -- in many cases because they never learned real coping skills or self-control. Sometimes it's the result of mental illness, but other times it's simply testosterone-fueled, childish rage at people seen as the enemy. The problem is magnified by access to two things: social media, where they can meet other losers to justify their motivations, and two, firearms.