Answer: Grendel is compared to Cain, the first murderer in Biblical history.
Explanation:
In Beowulf, Grendel is a creature that continuously attacks the mead-hall of Heorot, killing its inhabitants and destroying the mead-hall. This is why King Hrothgar invites Beowulf to save Heorot, as Beowulf is the only one who is not afraid of Grendel.
Grendel is described as the descendant of Cain:
''He was spawned in that slime
Of Cain, murderous creatures banished
By God, punished forever for the crime
Of Abel’s death."
Cain is the first murderer mentioned in the Bible that murdered his brother, Abel, out of jealousy. Both Grendel and Cain are miserable creatures. Grendel's mother is described as one of the 'fated ghosts' that spawned from him, and as a result, she is not desired in human society.
