Sure, if you really really really want to kill a bunch of people you can get guns even in "safe countries". Hell, at worst if it would be literary impossible to get guns you could go to the supermarket and buy fertilizer and stuff to make a do-it-yourself-50-virgins-in-paradise-kit.
However, a lot of such incidents are essentially cases of "temporary insanity" which can pass. It is a bit like with suicides, suicide rates go down when it is not that easy to get "fire & forget" suicide methods like various pills. And, surprise, we also have fewer homicidal mass murder cases per population in countries with gun control. The point isn't to make abusing one impossible because that in itself is impossible, but to make it harder. The effect of :effort: not only applies to cleaning up your room, but also to murdering everyone near you.