why is grendel considered the "descendant of cain" in beowulf? how does it relate to the biblical story?

Question

Why is he referrred to as the descendant of cain?
can someone expain to me the story of cain and abel?
i understand htey are the sons of adam and eve, and cain killed abel, but how does that relate to Grendel?

Answer 1

Cain was cast out into the wilderness and was more or less struck from human society. Though not so much now, in the past he was much vilified as a monster. Grendel as a descendent of Cain, while probably a Christian gloss on an originally pagan story, would have emphasized his alien nature to the people of that time.

Answer 2

The nature of Grendel's identity is something of a conundrum due in large part to a line where he is described as descended from the biblical Cain, the first murderer. For some scholars, this justifies a monstrous appearance. For others, it positions Grendel as a marginal (rather than monstrous) figure which bears the Curse and mark of Cain.
However, the epic also defines Grendel as a jotun (or giant), i.e. a troll. Within the poem, he is described by many different terms including ent, ettin, and "scather" ("one who scathes"). The terms ent, eoten or ettin, meaning "giant", were synonymous with "troll" in Norse mythology (see e.g. Nafnaþulur), and clearly connect Grendel with the Jotuns/trolls of Scandinavia.