Angels are Spirits with the power to take a physical form on Earth in order to interact with the physical space. Just like how satan possessed the serpent or the 3 Divine beings that came in the form of men and visited Abraham before the destruction of Sodom and Gomorrah, those men ate with Abraham.
But the Authorization for Spirits to mingle with men is only given by God and since those spirits used illegal entry and committed atrocities they became Fallen.
Yes, the sons of God were angels, they became fallen when they broke protocols.