Who is Zane/Death/Thanatos from On a Pale Horse and what is their importance?

Zane is at the lowest point in his life at the beginning of the novel, and it's also the end of his life, in a way. Some time previously, Zane's mother was in the hospital, sentenced to death by a terminal illness. Worse, she was sentenced to continuous pain with no promise of salvation by expensive life-support systems that had already depleted all her savings. She begged her son to help her die, and he did. This memory is the key element of Zane's character. He is both overcome by guilt and driven by compassion. He is willing to sacrifice his soul and to sin in order to alleviate suffering for fellow human beings.