[TigersEye]

Hoe did this whack-job ever pass an FBI background check for his gun license?

That is simple. He had never been convicted of a felony and had never been legally adjudicated mentally incompetent. That's pretty much all a NICS check will turn up.

[TigersEye]

Oh, one other thing will cause you to fail a NICS check to buy a firearm. A restraining order for domestic violence. No conviction necessary.