What is the main conflict in For Love of Evil by Piers Anthony?

In a world where magic is as powerful as science, Zane is a balanced soul: half good and half evil. When he decides to kill himself, Death comes for him to take and judge his soul, and Zane shoots Death instead. He discovers that he must take Death's place, and further, he discovers that he's been manipulated by supernatural powers to save the beautiful magician's daughter Luna and foil Satan's plans to gain control of the world. Zane breaks all the rules of Death, going on strike and warring against Satan. He emerges victorious, having earned Luna's love and brought compassion to the role of Death.