Demons are the Pagan Gods of Ancient Times

[Byrd]

Demon's and Angel's are two sides of the same coin. You can't believe in one without believing in the other.

I look at them through the lens of history as teaching tools used by religion and government to keep the masses under control. Stories of Demon's and Angel's are and were a way of scaring people to get them to act right.

Late to the party, but yes you can believe in demons and not in angels.  All sorts of old Mediterranean religions believed that bad things happened to good people because of demons -- but did not believe in angels or angel-spirits.  The demons (generally) weren't commanded by a god but just wandered around hurting people.