I would not vote for the death penalty in this case. For one thing it costs more to execute someone than to imprison them for life due to the appeals process. Also, killing him will make him a martyr in the eyes of many.
While what he did was horrific and unforgivable, but I do think he was influenced by his brother and due to his age might not have understood the complete, full ramifications of what he did, plus he was a member of an extreme religion, and we all know how religion can warp your values. His apology does not ring true to me, but I feel spending the rest of his life in prison will give him time to reflect on the path he chosen and maybe come to feel true repentance. He can think about the lives he took, about the life he might have had and the wife and children he will never have.