1. I generally take the functionalist view towards the Holocaust.
2. Hitler too was the product of his enviornment.
No one can create their own enviornment, and if you think that's how it works you need to re-evaluate your way of thinking.
He doesn't have a chance for rehabilitation, not because he isn't capable of changing, but because everyone around him won't.
"So what if he'll probably be killed by angry mobs after release, and if not, his status as a social pariah may drive him even further insane, that's his problem"
I see no certainty in these things and I seriously doubt the people of Norway are a bloodthirsty lot to do so, given the luxury he was awarded with.
He'll never get free. It might seem like Norway are very soft on their criminals (which is honestly the right way, if they rehabilitate that means they won't come back and will stop wasting your tax money), but once everyone in the system is against you, the system itself won't save you. Breivik will be locked away from society for good. At that point you shouldn't care whether he's dead or alive.
He's like a real-life Joker.
That dude had an entire wing of Arkham built just to house him.
Especially after you slay 77 members of your species.
He had rights to life, liberty, property, and travel like all free men of this world and he chose to use them to take away those 4 rights from people. 77 People.
Talk about rights and wrongs, the right thing to do is kill him for his wrongs.
Also life's short and sucks. Why not try and maximize happiness when everything around you is shit?