I believe demons are not fallen angels. I believe they are the spirits of the dead giants from the ancient world, or Nephilim, or Raphaim. Fallen angels are a different class of creature, more powerful and more dangerous, to be dealt with by asking God to rebuke them, whereas demons, we may rebuke directly using the Lord's Name and authority. Demons are actually part angel because the giants were part angel and part human. If you were to ask any 1st century Jew what are demons, he would say oh that is easy, spirits of the dead giants, it was assumed in the ancient world that was true. That belief started to change with Augustine, he was the first to introduce the "sons of Seth" theory of Genesis 6. Augustine was a genius and a spiritual giant, but had weak spots like any human being.