What experience led Jay Christos to become a Benandanti in the novel, VenCo?

Over the course of the novel, we learn that at one point, as a human, Jay had been in love with a woman named Prudence, who was also a witch. Though he loved Prudence deeply, she betrayed him. This caused Jay to live the rest of his life resenting her. His experience also led him to become a Benandanti to exact revenge on witches.