The "good guy guns" argument comes out every time people discuss gun control.
This argument, broadly, suggests that law abiding citizens using guns to defend themselves is necessary for the safety of our society. The problem with the argument is not this logic, it is how this logic is misapplied to argue against stricter gun laws.
Strict Laws don't stop the "good guys"
We aren't banning guns outright any time soon, if a "good guy" seeks a firearm to defend himself, he can undergo the background check and wait the waiting period. There's no reasoning as to why stricter gun laws at all reduce the ability of a "good guy" to acquire a weapon for self defense.
Not all firearms are created equally
Assault weapons are designed and built to offensively take human life. Are good samaritans pulling out a full-auto AK-47 with an extended magazine to stop a mugger coming at them with a 9mm? Do you load up your glock with teflon coated "cop killer" bullets (banned in only 7 states, conditionally in 2 of those) to defend yourself against a home invader? What about a automatic shotgun with a 30 round mag? These are all legally purchasable in the united states, and I'd argue that these guns are rather obviously doing more harm than good in society. The only justification for owning military assault weapons is that they are fun, weigh that against mass shootings and choose for yourself.
Some might argue that these weapons are necessary as a "check on the power of the state", but in this day and age that is just a joke. You gonna stop the government from driving an armored personnel carrier through your front door while watching you jack off with thermal imaging, with a rifle? Obviously the public can't be allowed to own anything powerful enough to really check a modern military state, so this notion has become obsolete. Guns aren't how we keep the government accountable any more, and we really should not want them to be.
Moreover, our poor mental health system in the US is NOT evidence for guns being innocent. Mental health issues certainly have had a role to play in many mass shootings and expanding the way that we care for and recognize those with mental health troubles is definitely a necessary reform. This does not discount the problem that many of those shootings are carried out with legally purchased guns, including the recent tragedy in San Bernadino.
Assuming the goal is to prevent mass shootings, doing so by identifying and stopping would-be shooters through a mental health system is nye impossible. That would require a fundamental rework of our health care system, one far more comprehensive than even what current single payer systems provide. In the case of mass-shootings, the perpetrators are often not criminals, with no access to the river of illegal guns that runs beneath every large American city. Stricter gun laws would in fact reduce these people's access to weapons, especially assault weapons with the greatest potential to take innocent lives in public spaces.
It should also be noted that the single most predictive variable when it comes to correlating firearm-related deaths is the gun ownership rate (http://www.amjmed.com/article/S0002-9343(13)00444-0/abstract). We may live in a society filled to the brim with guns (more than one per man, woman, and child in America), but that isn't a necessary reality.
EDIT: One final thought: mass shootings are a relatively new phenomenon. They started slowly in America, and now we are at the point of having more than one every day on average this year. The media won't stop putting these psychopaths on a pedestal of hatred (giving them the attention that they crave), and so new shooters won't stop appearing. Its getting worse; part of the conversation at this point has to be about damage control.
TL;DR: Our current notion of the second amendment (any guns, any time, for any one) is beyond absurd. Restricting gun ownership is not a constitutional violation.