I don't think you can class the likes of Breivik with common criminals or run-of-the-mill manslaughter/hot-blooded murderers, or even professional killers. Breivik is a self-professed enemy of society, he sees foreign armies all around him which he is entitled to fight with lethal force. There is no solution for that but to keep him away from anything and anyone. This is not even punishment, this is simple protection of the species. Even if there were a cure effective 99.99% of the times, society simply cannot afford the 0.01% risk.

By all means treat him as best as you can, but he just can't be released in society anymore. The best you can hope from him is that he becomes a great monk.