You are strongly mistaking "Psychiatric Ward for possibly the rest of his life" for "Getting off scot-free".
He is going to be behind lock and key, heavily medicated and very likely personally restrained for the rest of his life.
Seriously, not kidding, the man will spend the rest of his life strapped to a bed for days on end (shitting and pissing himself day in and day out, i need to stress this, when they strap you to a bed, if you need to shit, you shit yourself. He is going to be spending a lot of time shitting and pissing himself.) punctuated by long 'interview' sessions wherein doctors will treat him as though he is a neutered monster trapped under a microscope.
Oh, and because he is clearly pretty fucking violent, he's likely going to be kept 'regulated' on Halperidol or stronger Haldol isn't one of the 'fun' meds. Any time this man has not under that microscope or literally strapped to a bed, he is basically going to be sitting in a corner, drooling at a wall.
His sentence is somewhat analogous to lifelong solitary confinement, except that he's going to be under the direct, physical care of probably like a half-dozen or more very large orderlies who probably don't particularly like him even a little bit. Even a little. (because btw each and every one of them knows exactly who he is. exactly who he is)
He has not gotten away with shit.
He is, however, clearly out of his fucking mind.
It is generally not a good idea to put clearly fucking batshit psychotics into a prison with other people who are, while possibly violent, are not likely to be batshit psychotic.
If you do that, people get killed. Either someone kills him or he kills someone, there is no option C.
Norway has chosen to medicate the brains out of this monster, strap him to a bed and stick him in a box under a microscope.
I feel this was the appropriate choice.