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.
The story of divine beings procreating with human women (Genesis 6) is expanded upon in the book of Enoch to tell how these angels also bring sin to humanity, causing the ancient flood as well, and this sin is the source of disease in the present day.