Devil is an older word for what we now call terrorist. Devils exist because they are terrorist by a different name.
Think you are onto something. Could one man's devil be another man's god? Like they say one man's terrorist is another man's freedom fighter?
NB "Man" in this context is short for "Human" before PC pronoun Nazi's get upset. Like "mankind" is short for "humankind".
"You devil" is something that can have at least to different meanings or connotations.
First, it can be used to for supernatural or fictional characters with evil actions.
Second, it can be used to call a person because of their evil actions.
Just because it can be used both fiction (or unverifiable supernatural terminology) and nonfiction doesn't mean the nonfiction use is invalidated by its fictional use.