The mind of a sick individual

Another tragedy. What is it with the world these days? A man sets off a car bomb in Oslo outside the Norwegian Prime Minister's office, and then goes to a summer camp - where friends of the Prime Minister were in attendance - disguised as a police officer, and guns down 68 people.
This guy then turns around and says that he did these things to defend Norway against Marxists and Muslims. Wow! When are people going to let these two groups off the hook? Muslims have been persecuted (along with other groups) since the dark ages, and continue to be the target of stereotyping and discrimination because of a relatively small radicalized few who carry out terrorist attacks in name of Allah (the compassionate, the merciful). Marxists have been the target of right-wing vitriol worldwide, and very specifically in the United States, since the Communist Revolution in 1919, and throughout the 20th century as countries throughout the world tried their hands at communism or socialism (they get lumped in as the same ideology, despite their differences). Although, I must say that all countries that have been described as "communist", have not actually followed through with Marx's vision to the endpoint, and have instead all stagnated in the "dictatorship of the proletariat" stage, with the Communist Party becoming a de facto dictator, with the head of the party as the head of the state (e.g. Chairman Mao, Premier Stalin, President Castro). But I digress...
What is scary about a man like this, and I guess people with mental health issues in general, is that their perspective of the world is so warped, that basically any and all other perspectives are at risk for offending them. This man described himself as being "at war", implying that what he's doing is essentially a crusade of sorts, and therefore he's not only right, but morally obligated to do what he did.
We've heard this story time and time again from fanatics with mental health issues (do all fanatics not have some mental health issues?), but I can't help but wonder why their mental state seems to so often manifest itself in religious or social intolerance and violence. What is it about our society that causes crazy (I use this word for ease, but I don't like it, and don't think that it gives the complexity of mental health issues their proper dues) people to often be crazy in specific ways? Are these people genetically inclined to be terrorist religious fanatics? Bombing churches, mosques, Planned Parenthood centers, or military recruiting posts. It's all just very strange to me. I cannot understand what makes these people do what they do, and I probably never will, I just thought it was an interesting thing to think about why they do what they do - or more specifically why they are the way they are - instead of just calling them crazy or insane, or some other euphemistic word that doesn't really do justice to the range of mental health issues that they may be dealing with.