Origin and Nature of Evil Spirits

Where do evil spirits come from? Second Temple Judaism had an answer, and it starts in Genesis 6.
1 Enoch tells it in two stages. First the giants are destroyed - they slaughter one another, and unlike their fathers, who are bound in the underworld, they are annihilated. Then comes the explanation of what happened next: the giants, born of spirit and of flesh, shall be called evil spirits upon the earth.
The logic is category transgression. Their nature was mixed, so when their bodies died, something came forth from them that had nowhere to belong.
Scripture links:
- Genesis 6:1-4 - the sons of God, the daughters of men, and the offspring.
- 1 Enoch 10:9-10 and 15:8-9 - the giants destroyed, and what their spirits became.
This is the background the New Testament writers assumed. It is not the Bible's own doctrine, but it is the world the Bible's first readers lived in.