Death, life without parole, heck, even just abandoning him on some deserted island (is Bikini Atoll still 'hot'?) would all serve one purpose. They would ensure that this monster is permanently sundered from the society he has rejected and that the society does not have to fear his return. Rehabilitation is an absolutely ludicrous conceit in the case of a deliberate mass murderer, and of children to boot. No penalty can be great enough to atone for his actions and no amorphous 'regret' on his part down the line could ever be believed.
Society, with little or no remorse, kills millions of unwanted pet animals every year. With cause, rabid or vicious animals are dispatched for the greater good. Just because the ravening beast walks on two legs instead of four makes no difference. Society should have the right to permanently remove him for its own safety and peace of mind. HOW its done really doesn't interest me.