What would you call a demon that wants to do good?

By the very definition of the word, there is no such thing as a "good" demon.

Also, since there are no such things as demons except in the minds of the superstitious and religious fanatics there is no reason to be concerned in the first place.

"de·mon
noun
an evil spirit or devil, especially one thought to possess a person or act as a tormentor in hell.
synonyms: devil, fiend, evil spirit