They should have never been in a position to cause harm in the first place, yet as of today, we still don't have any functional mechanisms that allow us to prevent incidents such as selling firearms to the mentally or emotionally unstable, probably because activists would cry discrimination. I'm not for banning firearms, but they should be far more controlled. There is no reason for a responsible, healthy adult not to own a firearm for his own protection, sport (hunting, target practice, etc) simply because others deem it unnecessary or feel uncomfortable. I am 100% convinced that if you disarm everyone, only criminals will be armed, creating an enormous demand in gun black markets, literally making illegal gun trade factions grow to unprecedented scales, making a situation that could be contained with minor regulation, into a worst case scenario. I've read the argument that the probability of needing a firearm to defend your home from a home invasion is so low that it is an unnecessary investment, considering the risks. I find this argument to be invalid for the simple fact that it assumes crime rates will remain constant, when in fact the economy of any country (as demonstrated in recent history) can go terribly wrong, giving way to mass growth of crime, therefore, increasing the probability exponentially of facing such a situation. TL;DR (regulate gun sales, don't ban them).
I'm not going to quote the posts challenging the notion that everyone is inherently evil (which I'm not sure why people consider this a religious idea), I will simply give my take on the notion, everyone has the capacity of doing evil, and sometimes it doesn't take much for them to use it.
Nothing to see here, move along.