If there is no good and no evil, there cannot be a devil. The very concept of a devil bespeaks a dualism that is contradicted by monotheism. If there is a good god opposing a bad devil, then that god cannot be omnipotent; both the god and the devil must be creatures themselves.
Historically, devils are inventions of monotheists. They are used as tools for the persecution of heretics and dissidents.
Here is a very good book about how the devil was invented, and has been used through history.