I'm inclined to agree with you on that. I was going to make him "bad" but not innately evil. He was going to do a lot of evil things in the book, but he still wasn't going to be completely evil.
A love spell or magic potion or some third party could explain the "wants to kill her but she marries the demon" plot. Or, the demon becomes more human through her love, and or she becomes more demon through his love, or they both become immortal or he renounces his "evilness" aka immortality for her proving he is worth her love. You will find a pleiade of such stories in ancient Egyptian / Asiro-Babilonian tales not to use the Greco/Roman ones that indeed (in my humble opinion) are slightly over used.
