That's not justice. While it may be unfortunate that some persons are crazy and end up killing people they are still the ones who did it and need to pay for their violent crimes against others.
To what end? It won't serve as a deterrent, nor will it protect society. Norwegian law speaks to having "a lack of consciousness" during the act or having a "permanently limited cognitive ability". If you lash out and kill someone during a psychotic break, "punishment" will serve society no more than punishing someone who accidentally drives into a crowd during an undiagnosed epilepsy attack.
In the first instance, they'd be sentenced to a high security mental institution until such time when they're not a danger to themselves or other, in the latter they'd probably never drive again. To some, that might not be as cathartic as vengeance, but it is justice.
And fuck that, if someone kills 90+ people crazy or not they should be held responsible. If this man somehow gets a lesser sentence or walks free for claiming mental incompetency/crazy
He won't. While he might be "crazy" in the sense that there's some severe personality disorder, there's no indication of psychosis or limited cognitive ability. And even if there was, given that he apparently had meticulously planned this for a decade of feigned "normalcy", I struggle to see how any psychiatrist could trust him to the point of declaring him "cured". Even after years and years of apparent recovery and reform.
At minimum put this guy behind bars for life
He risks up to a 30 year determinate sentence or a sentence of "preventive detention". The latter can be renewed in five year intervals for an indeterminate amount of time (after the initial period of up to 21 years) if the detainee is he is still considered a danger to society.
The biggest problem with locking him up post sentencing will probably be where to put him. Norway have exactly one enhanced security wing in a prison where there's the long term facilities to completely isolate a prisoner from all other inmates. And they might not be able to put him there if it interferes with the operation of the unit with regards to its normal use. I suspect there for many years will be a high risk of him being killed if other other prisoners are allowed near him.
if there's no death penalty in Norway, time to make an exception.
No. Changing one of the pillars of a justice system (which in most ways works remarkably well) in such a way in response to the acts of one evil man would only let him win.