It's painfully obvious what replaces the gun. Teaching, nurturing, and environment.
Many people are absolutely trustworthy, even with nuclear weapons at their fingertips. Other people are so dangerous they have to be locked in cages, with extremely limited, controlled levels of contact with others.
Granted, some of the difference is related to dysfunctional brain processing, such as genetic defects, or psychological / physiological trauma. Another problem is unnecessary stress. Many animals will begin lashing out at those around them if enough stress is introduced into their environment.
The only argument for a gun ban (the goal of all gun control efforts) is that guns are immediately available, and most killers claim that, had they been able to "cool down" they would not have acted. That's true most of the time after a homicide -- no doubt the pressure cooker perp in Boston would say the same thing. Or the knife, club, and stone crowd. A lot of animals feel confused after a lash-out incident. Generally, they weren't expecting the cascade towards rage to creep up on them in such an insidious way.
The worst solution -- as usual -- is the one being proposed by the progressive democrats and the gun control advocates. It's childish flailing at a serious issue, the worst possible way to find a solution. Gun control always "progresses" to a ban, and they keep denying their true intentions -- they are liars. Gun control is rarely, if ever, a factor in murder of any kind. It's easily bypassed, as seen in the myriad nations that have complete gun bans, or with the substitution of another tool, such as a stone, club, or box cutter. Other nations with total gun bans often have homicide rates far in excess of any level seen in the US.
The US homicide rates are as low as they were in the 1950's, well before NICs, and well before the peak seen during the reign of Bill Clinton and Oprah, when the people of the deep blue, democratic party strongholds were lashing out at each other in record numbers. As we saw the other day, the current President blames all of America, not the perpetrator. This is a common trait among progressive democrats.
Mental illness -- at least the easily detectable kinds -- are also a negligible issue. Many mental health professionals have warned that sensationalising these stories, and blaming the mentally ill, just makes things worse. In the midst of the Confederate flag absurdity, two doctors said that it was a statistical coin toss -- about the same as the general population -- as to whether any of their patients would harm another person. The NICs system, developed in the 1960's by the two parties, and the NRA, already has several paths to removal of the 2nd amendment right. Somehow, the progressive democrats believe that more is better.
Far more likely is the sad reality that we are living in a time of a primitive, 6 B. strong human primate infestation of the planet. Most have never seen a science book, or any book, other than a religious text. The interpretation of the teacher is often anti-science, anti-reason, and anti-America. Many human primates are being pushed and prodded to compete, sometimes violently, and consume greatly, for the benefit of a small group of self-proclaimed wealthy elites.
If we can find a way to destress the human primate population with low-cost, widely available food, sex, and interesting activities, a lot of stress of any kind would go away. Widespread food production, easily obtained sexual gratification, interesting activities for human primates (posting opinions on slashdot, etc) should dramatically reduce the desire to lash out on the innocent. Not to zero, but a great improvement. Welcome, Internet.
The sad truth is we've spent trillions on "great society" programs, we have a horrifically expensive education monopoly, we have junk products everywhere, poverty rates that have remained stagnant or increased for every demographic other than the elderly since the mid-1960's, angry groups and individuals lashing out with words and deeds, and the media shows all of it to us, all the time. They call it freedom of the press.
Teaching people to step away from stress should be job #1, right now. There are simple techniques to reduce stress, they are well known and published, and have been for decades. For the vast majority of human primates, living without the basic necessities, stress reduction is only an unattainable dream.
It doesn't matter if a person is a cop, a perp, or suicidal. These discussions ignore the underlying issue.