"Demon" is a label put on something. The true back story on these things probably has little to do with our mythologies about them. However, for all practical purposes these things largely fit the description. Are they angels cast from heaven or whatever. Probably not. Are they something that those stories were based on? Maybe so.
Whatever these things are, if they are real in any sense of the word, they are evil from a human standpoint. They allegedly routinely violate the privacy and persons of individuals and commit acts of rape, terrorism, and kidnapping if the reports are to be believed. Not good. I don't care if they are alien scientists, doctors, or genetic engineers working for our greater good or some such.
There may not be a true EVIL as humans conceptualize it, but there are surely things that are destructive and counter to our well being we could label as such.