Consider this for a moment, "You are not your parents" Being an angel or a demon, in my belief any way, is just a matter of your birth. Saying a demon MUST be evil in your story is like saying a Jewish character in your story MUST be a cheapskate or a gay male MUST be super feminine, it's stereotyping. In my book, one of my characters is an angel demon hybrid. Although yes, his father was an evil demon, he himself is not evil. If he can maintain in inner balance, there is no reason these things are impossible. His son his a cross between Angel, Demon, and human, and he is one of my main heroes. Balance is possible
If you want inspiration for this type of storyline then you might want to read some Judiac, Gnostic and/or Islamic stories about the prehistory of the world. Some of the stories about The Fall are full of drama and inspiration that you could easily borrow for your plot line. And yes, there are some very good reasons why Angels and Demons would work towards the same end.
