You know what kicks me right square in the nuts? That people group "mass murders" together as if they were all one unified thing with a single origin and single set of circumstances causing them. Ya wanna know what this was a case of? Incompetence, on many levels and from many people, including the killer himself. People dropped the ball, likely because he was a well-to-do white kid and we all know that the ones that go on violent mass killings are scary poor people and minorities. He didn't have a record because he was, again, a well-to-do white guy, and you don't punish the good little boys for getting a little angry. Besides, as was pointed out before, a gun was only one of his weapons, and YET, no one is giving the fucking car the stink eye or saying we need tighter knife regulations.
Its horrible, but some people, you just can't fucking stop from going on a rampage. If you take the gun, they'll just use something else. Maybe he wouldn't have killed as many people, maybe not. Its like when the shooting in Jonesboro, AR happened. Everyone was all over enacting new laws to restrict weapons, as if that'd have stopped them. Mitchell Johnson and Andrew Golden stole their weapons from Golden's grandfather, along with their ammo. We already have laws against theft, but that didn't stop them.
Determined criminals will break the god damned law. Determined murderers will kill people with or without firearms. Do they make it easier? Sure, but a knife makes it easier to kill someone when compared to beating them with your bare fists, too. This motherfucker was god damned insane, not having a gun wouldn't have stopped him and I highly doubt it'll deter any others of his ilk. America's got a lot of people, and a history of looking the other way when Mighty Whitey is around.
The police looked, but they did not see. Because they didn't want to see. They didn't do anything because he was an affluent white kid, and no affluent white kids have ever been problem children. The problem is not one of regulation, but one of enforcement. Incompetence fucks over everyone...and sometimes, incompetence kills.