What made Cain capable of murdering his brother? Why was the flood generation so wicked? According to Pirqei de-Rabbi Eliezer, the fallen angel Samael embodies the serpent and seduces Eve, whereupon she conceives Cain. Engendered by this “bad seed,” all the descendants of Cain become corrupt, destined to be wiped out by mighty waters.
According to the non-biblical book of Enoch, Genesis 6 tells of angels who bring sin to humanity, causing the Flood as well as sin and disease in the present.