I mean, I have a theory as to the root cause: entitlement.
This guy thought he was entitled to do whatever the fuck he wanted to do, just like the mass shooter he shot, and that the rules don't apply to him.
If he brought the gun to the mall that day, chances are he's brought it to that mall many times before.
And every time he brought that gun to the mall, he wasn't doing it because there was a mass shooter lurking -he couldn't have known that unless he was in on it- he was doing it to feel like a big man for thumbing his nose at authority.
That doesn't make him a good guy...it makes him a bad guy.
I'm just going to let this insanity by this nutcase speak for itself.