I agree that even law abiding individuals who are rational and/or sane can change and, in a moment of rage, use a gun against another (innocent) human being. They can also use a knife, a machete, a hammer, a baseball bat, a chainsaw, a frozen leg of lamb, etc. The damage inflicted would (at least on average) be less severe if a gun was not used.
Omar Mateen and Adam Lanza were obviously not law abiding citizens even before they opened fire (and neither was the Colorado movie shooter). They all broke the law when they entered gun free zones where they then committed mass murder. It's difficult to believe that any of them were mentally healthy. It is also difficult to believe they were insane. They obviously planned their actions in advance.
Even if it was possible to get all law abiding citizens to turn in their guns (if it became the law to do so), how long do you think it would take to get the rest of the guns out of the hands of those who have no desire to obey the laws regarding having, stealing or using guns?
The British MP was reportedly shot with a homemade gun. If that's true it shows that it is possible to have a gun without buying or stealing one. It has also been reported that the man who killed her has mental issues, but if he actually had a homemede gun, he probably knew it was agianst the law to make or possess it.
http://www.fromthetr...un-knife/162486
PS: Homemade guns don't have to be primitive, either. With 3-D printing it is now possible to create a modern, homemade gun that works:
http://www.bbc.com/n...onment-22421185