It's a difficult one, at what point does a superstition become a compulsive disorder? Remember that for something to be a disorder/disease there has to be some health implication which has huge grey areas. It sounds simple sometimes but it's really not. I remember a lawyer friend of mine telling me about a case some years ago (back in the 80s/90s I think) of a homosexual S&M club that was raided by police. The youngest member was just 18 and was into having certain body parts stapled to furniture. The judge apparently said that the person clearly had to be mentally ill because of this decision, in actuality he was just into some pretty bizzare but not health harming activities (the acts caused minimal/superficial damage). The problem is that some people find it extremely difficult to imagine how someone could choose to do something that they themselves find a crazy choice.
With regards to Breivik, taking in what I know (which is not of course the full story so I may be wrong) he is perfectly capable of rational thought and planning. His actions were decided by his extreme ideologies that are not necessarily the product of a mental disorder.