And as this wiki article posted in other comments very nicely explains, Tolkien never came to a good and final conclusion on how this all really worked, with different explanations in different works of his. The "they were just evil force that could be killed without remorse" theme is the dominant one, because it works in the context of the story and the worldbuilding that he did for it.
https://en.wikipedia.org/wiki/Tolkien%27s_moral_dilemma