Here is a small bit of info from the web.
What is the full meaning of devil?
the spirit or power of evil
devil, (from Greek diabolos, “slanderer” or “accuser”), the spirit or power of evil. Though sometimes used for minor demonic spirits, the word devil generally refers to the prince of evil spirits and as such takes various forms in the religions of the world.
So it's completely fine to use devil to refer to a demon or evil spirit. Happens a lot in old literature.