The term "Demon" comes from the ancient Greek word 'daimonion'. It is believed to be later shortened to 'Daemon' or 'daimon' all meaning an entity or deity of supernatural origin. This is to mean historically both gods, monsters, or any positive and negative spirits were indeed defined as daimonions.
When Catholicism swept through Rome the Catholics saw these beings not of their religion nor of their "god" and saw these beings as evil. As this monotheistic ideology spread more and more saw these daemons as evil, and thus as culture more generations saw these gods as evil. This happening more and more these beings eventually became demonized as lesser and unholy beings. This happening more and more (at the time) pop culture spread this with literature, art and eventually movies and televisions until this came almost obvious to new generations. although they are not, they were beautiful and powerful gods.
Again I mean this from the view point of history and how us humans eventually as a large saw it. It is sad, but we cannot change it none the less. Once again I did not mean to offend anyone with this information, i apologize if I did.
I hope you all have a lovely day, thank you for reading.