maybe someone from Norway should have a say here
Almost everyone here is happy with the judgement. Please do not confuse the word sane with reasonable or good sense. It means, in this case, that he is responsible for his actions and therefore can be punished by imprisonment. 21yrs is the maximum allowed. But he will never be allowed out because he will always be classed as a danger to society so his term will be extended again and again. The other option was to class him insane and therefore he was not responsible for his actions.
He was clearly very aware of his actions and very responsible for them and therefore he has to be classed as sane in this sense.