First of all, hello to you all! :)
If the info that's coming through proves to be correct, I think I can see his rationale. In his mind, it was all (to use a typical liberal-baiting American conservative expression) the lefties fault. The way he saw it, the fact that foreigneirs, non-Christians and basically everything typically non-Norwegian was entering his country was all due to the Left's policies.
Bottom line: he was a man driven by hatred borne out of frustration of some kind. I mean, he was a suburban, middle class man in his mid-to-late 30's "still living in his momma's basement" (another typical American expression here). He was obviously someone with antisocial issues. And as we all know, nutcases aren't exclusive to any social class, nationality, race, religion or political affiliation.