It is always hazardous to pronounce on the mental state of someone one had not met, and about whom one knows only a little and third-hand. But all the same, one is tempted…
The first trap to avoid is to say person x did act y because [he] is or has z, and we know he is or has z because he did y. This is circular.
But there does seem to be evidence that Breivik was narcissistic, grandiose, paranoid, socially and sexually inept, and deeply resentful. This is a horrible mixture, though any explanation will always be incomplete and not pluck out the heart of his mystery.
I think it unlikely he is legally insane according to the M’Naghten rules that govern legal insanity in a lot of the English-speaking world. He knew the nature…[a]nd quality of his act and that [it] was (legally) wrong, to use the wording of the rules, and therefore would not be entitled to a verdict of not guilty by reason of insanity. (Posted at The Skeptical Doctor on July 26, 2011.)
Hitler and Stalin often are called “madmen” and “insane” because of the utterly depraved acts that they ordered and condoned. But they were not insane. They were evil.
Evil should not be allowed to hide behind the cloak of insanity.