1) Some mentally ill person completely ignores existing gun laws and shoots a bunch of people---unless one of those people also has a gun and knows how to use it, then it never makes the news.
2) The nation mourns the losses.
3) Demands are made for meaningful gun control legislation.
4) Firearms sales go through the roof.
End result---more guns in circulation. New laws enacted that only the law abiding folks pay attention to. Root problem of mental illness not even addressed. People who were already very well armed have more guns. That important margin of good people who might be present to step up, with the ability to properly administer lethal force, try to find some middle ground instead.
Guns are not the problem, just the method. As long as we as a society cheapen the value of life, and treat it as something individuals do not have a personal obligation to protect---be it a pregnant unwed teen, festering gang activity, drug and alcohol abuse, or groups of children protected by only unarmed adults, this sort of thing will increase in frequency.
A good friend works with the 'frequent flyer' 911 abusers. It's very clear that what's missing is proper mental health facilities. A great many of the homeless are simply incapable of fending for themselves, and use 911 as a way to easily get some level of intervention. What's needed is good profiling of citizens---citizens identified by parents, family, teachers, medical professionals, and law enforcement. Make these people feel like they matter, like their life has worth, and it will do far more to prevent these tragedies than banning some particular form of firearm or some capacity of magazine. Identifying the mentally ill people who actually pose a threat to society and taking custody of them is a reasonable infringement of liberty, but the groups that fight that are often the same groups that have no problem infringing upon responsible firearms owners.