In this case, the shooter did NOT have a criminal background. So he would have (And might have, for all that I, we, or anybody else knows.) passed a background check. There are problems with gun control, but the NRA didn't have anything to do with this, they're just an easy target for politicians.
IMO, screw bump stock kits. I don't even want guns that can be upgraded with them polluting up my world. These guns are created for one purpose and that is what happened in Las Vegas. Yet we complain and cry when it happens.