In the tale of Beowulf, the monster Grendel is said to be descended from the biblical character of Cain. This was surprising to me, as I'd thought Beowulf was a pre-Christian story given its setting in pagan Germanic countries, but I learned that paganism vs Christianity in Beowulf is a hotly debated subject, so I won't go into that large topic here. Instead, my question is simple: what is the significance of Grendel being descended from Cain? Does his ancestry have any bearing on the character of Grendel, or is there any parallel between his story and Cain's? Why does the story mention Cain as an ancestor of Grendel?
The story of the descendants of Cain is told in Genesis 4. Reading it may clarify this passage in Beowulf.
Cain was an outcast for his crime, doomed to permanent exile. Jehovah sets a mark on him that somehow protects him and anyone who kills him would be avenged sevenfold. He marries, though the Old Testament does not explain where these other people came from, since Adam and Eve are the first people. Maybe they were created by some other god? In his exile Cain is the founder of a city, and his descendants are great artificers. In the fourth generation a man named Lamech confesses to a murder. Lamech makes this cryptic statement to his wives “… I have slain a man to my wounding, and a young man to my hurt. If Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold.” The story of Cain and his descendants ends there, with more murders and threats of many more.
It was my understanding of the passage in Beowulf that it was a literary allusion. Cain was not literally a progenitor of Grendel. Grendel, like Cain and his offspring, was an outsider and murderer.