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

Sin is the major theme. Death's main role is to weigh the balance of goodness and sin in any soul that he takes. Zane, in his role as Death, has many quandaries about the sins he finds on mortals' souls. Zane must take the souls of babies who are marked with sin through being born of incest or rape or other sinful acts. He also learns of sins that were committed for reasons of love and loyalty, like Luna's sin of summoning the demon.