He has to be treated humanely. Otherwise we just lower ourselves. But he is entitled to zero sympathy. I can think of no good reason to allow him the company of others - any others. There is also the risk that he could radicalise others and I would hate to give him that option. If he is stuck with his own twisted company for the rest of his life, which I hope with all my heart and soul will by long and empty, then so be it. Give him the Qur'an for company.
I think that's the issue. That it's inhumane to completely isolate someone indefinitely. I really don't know the solution.