These mass shootings almost always happen in places where people DON'T carry guns or have security. People don't carry guns and there aren't security guards or cops in churches or schools. I think that there's almost universal agreement that the problem is not guns themselves (Otherwise all gun owners and cops would be shooting people.), but guns getting into the WRONG hands!
People don't carry guns nearly everywhere, and schools do have security. Most people committing these crimes WANT there to be security so they can go out in a blaze of glory or get their 15 minutes once they're caught.
IMO, this latest incident pretty much proves once and for all that the problem is not background checks and wait periods. These systems are designed to fail. The problem is having these machines of death available AT ALL. Unless you are part of the military or law enforcement, you have no business whatsoever with an AR-15 or similar weapons.