(I'm a novelist.) Is it sinister for Christians to write about demons?

In my own opinion, no. I say no because so many people, Christians included, don't realize or downplay just what a demon is. It is a spiritual being and attacks people on a spiritual level. They don't hiss and spit at you, rather break you down mentally. People should have a basic understanding of what demons really are and how they can overcome them. So no, it's not sinister.