The sentence could be extended, potentially indefinitely, in the future if he is considered still to pose a threat to society. Norway does not have the death penalty.
Per the article.
Just different set of rules. They don't in general encounter this kind of thing much. I've been there many times and always felt safe. The max penalty is what they gave him. Had they agreed he was insane he'd get less time and only go to a hospitAl. I agree he should be locked up longer but that is unfortunately the law. Still if the time comes maybe they will just keep extending him if they still consider him a threat thereby effectively being a life sentence anyway.