Your post supports the gun culture which perpetuates the problem. The "good guy with a gun" slogan must have been an NRA PR thing.First, I fully support background checks on gun purchases similar to the ones on NYC. That being said, it won't help in the cases of lunatics. Sandy Hook shooter used his mom's guns. No background checks would have prevented it. We can put restrictions on high capacity magazines and things that turn semi into full auto, but it does not solve underlying problem.
Even if 2nd amendment repealed and all guns confiscated (won't happen), what would have prevented this lunatic renting a big truck and killings kids leaving school?
Unfortunately, the only solution is: to stop a bad guy with the gun you need a good guy with the gun.
Rather than look at all of the other ways that a psycho can kill people (trucks and so on), consider the manner in which the majority of these head cases actually dispatch their victims. And then address it.