First, people need to stop using Norway as an example...that place has like 5 million people...not 315 million people. I am going out on a limb here and I am going to guess that while Norway has less shooting than the USA...that the USA has more doctors, nurses, lawyers, PhD's, people with graduate degrees, more engineers, more teachers (Just tossed that in for me) more of everything that is also "good". You know why? Because America has more people.
To help out on what @JJ Husker said and to help beat down some of the "where are the facts" crowd...well PTSD was not "a thing" until 1980...until that became a thing you were just not "tough enough" or you had "shell shock" or you were "going through some things" Now, we realize that it is a real thing.
If you use a gun, knife, bat or anything to kill someone...you have an issue.
Ted Bundy refused to ever admit he was "nuts" he even went as far as to say he was insulted by his outside council when they said he should try using that as his defense. I don't think it was until 86 where a doctor finally said Ted was bipolar. We all know Ted was nuts.
Crazy doesn't need to fit into an already determined box, just like it didn't for people with PTSD until 1980. The "facts" crowd in 1979 "Oh that vet just needs to get their act together, nothing is wrong with them" the "facts crowd" in 1980 "Oh, that vet needs mental help, he has PTSD"
The greatest trick the devil ever pulled off was convincing people he didn't exist. Evil to that magnitude is a result of mental illness in one way, shape or form. It doesn't mean it should not be punished. It should. But some people are born bad and others become bad.