Justice, not what he wants, is what should matter. On the one hand you have a painless, humane death. On the other hand you have years and years of sitting in a cell, 23 hours a day, solitary confinement, no touch with the outside world at all. To me that would be sheer hell, and I want Moussaoui to spend the rest of his life in our hell before going on to the other one.
I respect your opinion but disagree. Our prison system is more humane than any in the world. He will be kept alive for many years at taxpayers' expense, access to books, mail, medical attention, 24/7 protection from those who would harm him, legal counsel, etc, etc. If he were to be denied any form of human contact for the rest of his life, I could accept that, but that won't happen.
To me, the deprivation of life is the ultimate punishment. But that's just me.