I think I like the mentally unstable people kill people argument better than guns kill people.
It makes it much easier for us to believe and accept. Because we now have an explanation for why someone did it. It is far harder for us to accept that someone willing made the choice to kill someone with no mental issues.
I know mentally stable people can kill people too, but it seems like to go on a killing spree or randomly kill someone for no apparent reason or because you are mad at the world there must be something is probably not right upstairs.
No one will know. The human brain is so hard to figure out. The medical world has a hard enough time as it is dealing with Autism, Bi Polar, and all the other illnesses.
Maybe for some people doing this releases endorphins and such. Makes them feel good similar to others who do risky things looking for that endorphin rush.