The accused did not "plead insanity." He was subjected to a mental test, because the assumption here in Scandinavia is that ANYONE who owns and uses a his gun to kill people is insane.
The court-ordered psychiatric evaluation resulted in a diagnosis of Paranoid-Schizophrenia, a very debilitating condition that causes delusions. In this case it is likely that the accused did in fact think he was doing society a favour by targeting the young people at a political summer camp organised by the left social-democrats.
The inspiration appears to have come from the extremely negative rhetoric of the far right, or at least that has been a large part of the discussion. Personally I am glad the whole "he did it because of a game"-angle did not take hold here. Some papers parroted the angle, but since education is free and the norm is at least nine years of school, most people know better. Fictional worlds may help enforce pre-existing delusions, but they are not the root cause of the deceased mind.
The murderer in this case, did not want a label of "insane," because it would completely invalidate his crusade against whatever it is he is crusading against. According to reports, he is disappointed with the verdict.
It is worth noting, that in Norway, as in Denmark a ruling of committal is in effect an open-ended sentence. No parole and no forgiveness. A relatively extreme sentence in a system where homicide is worth 12 years and most crimes carry a maximum sentence of between 4 and 8 years. Also, in most cases, we tend to throw people away according to the worst thing they did. So a murderer who stole the victims purse might get the 12 years for murder, and not the 2 years op top for theft.
As you can probably understand, the mass-murder of politically active individuals is viewed as a direct attack on the foundation of democracy. The accused has attempted to reason with his accusers, and he has failed. He is now at the mercy of doctors, who will not let him out of their sight for the rest of his life. IF he is ever judge normal, could in theory be released before he would have been, had he received a conventional sentence.
As pointed out above, prison is better that an open-ended committal to an insane-ward. In prison, he would likely be left to evolve his decease, and after serving his time, he would get out and likely cause more problems. With doctors around him, he will be forced to either see what he has done and begin suffering properly, or he will stay there until he dies.
Paranoid-Schizophrenia usually shortens the life a lot, so he is unlikely to last long.