The devil originates among the Essene Hebrews and really takes off among the early Christians – Luke in particular.
The devil thus represented all that was evil in one’s eyes and in the eyes of whatever God one imagined serving.
The devil was in the eye of the beholder.
Whatever you hated was of the devil.
Of course you’ll find this definition offensive if you believe the devil and the god you worship are real and if you think they’re real you probably think they’re the most real things in the universe.
So a further refinement.
The devil is a subjective fiction that his inventors claim is among the most real forces in the universe.
The devil is a subjective fiction claimed as objective fact – fake objectivity – subjectivity pretending to be objective.
If you’re very religious you probably resent this fake objectivity in your religious rivals who clearly don’t know how to tell your objective truth from subjective fiction.
If you don’t trust religion at all you probably regard this fake objectivity as one of religion’s worse features – the source of a whole lot of religious people calling other religious people the devil’s spawn throughout history and therefore an idea whose time has passed.