It made perfect 'sense' for him to do that. He blamed the political elite for allowing immigration to occur. So he attacked the source of the 'betrayal' - the Labour party and their children, to 'teach them a lesson'
Of course the sociopathic nature of this coincides with whatever mental processes he possessed. But this was his reasoning. He believed and continues to believe that he is a righteous warrior in a grand historical narrative. That this was an act of self defence of white Norwegians in a war.
He's not a white nationalist, he's a western nationalist, there are subtle differences yes, but it is important to distinguish between ideologies. Just because someone can lay their thoughts in steps doesn't mean they are acting rationally. If I wear a tinfoil hat because I believe aliens can read my thoughts and tinfoil will keep them out, society will consider me insane, as will a court.
The legal issue here though is whether this can be considered legal insanity, which is poorly defined at the best of times. This article is interesting
http://legal-dictionary.thefreedictionary.com/Psychopaths+and+SociopathsIn it it states that in the US a sociopath would be unlikely to meet the criteria for an insanity plea, but this sort of crime is unique in Norway, this is the worst crime they have ever seen in modern history, so this is unprecedented, we'll see what happens, I have a feeling he will be declared insane. Plus, unlike the reactionary self-serving justice system in the US, in Scandinavia and Norway in particular they have a more clear-thinking justice system which recognises pre-determinate factors and the sources of criminality (other than just because of 'bad people') so I have a feeling that their understanding of insanity will vary from what we are used to.
Oh and abood, I hate all of them, but comparing their ideologies is like comparing me (as a social democrat) to an anarchist or a PETA-inspired terrorist. When it comes to politics we have to recognise the differences, subtle as they may be, generalising is a bad idea.