Why does Beowulf slay Grendel in Beowulf?
In the anonymous epic poem, Beowulf, Beowulf comes to King Hrothgar's aid. After he arrives he says:
Hrothgar can hunt, here in my heart,
For some way to drive this devil [Grendel] out--
If anything will ever end the evils
Afflicting your wise and famous lord.
By the time Beowulf arrives to help, Grendel has been killing Hrothgar's men for twelve years. The mead hall is nearly ruined. In the end, Beowulf's method of helping is to kill Grendel himself.
Of course, being an epic poem, the narrative goes deeper than that. Grendel lives "...down/In the darkness,..." He makes "...his home in a hell/Not hell but earth. He was spawned in that slime,/Conceived by a pair of those monsters born/Of Cain, murderous creatures banished/By God, punished forever for the crime/Of Abel's death."
Grendel is evil, Beowulf is good. And good must triumph.
Interestingly, the poet juxtaposes this material about Grendel's evil home and ancestry with descriptions of music, rejoicing, song, etc., in the mead hall. It appears that the speaker is associating musical celebration with the monster's reason for stirring and his eventual attacks. If so, Beowulf's victory might also be seen as a victory for art.
In the epic poem Beowulf Grendel has been killing Hrothgar's people and terrorizing the King. Beowulf is told by the Geats of the tale of Grendel and that he is a strong monster. Beowulf is a larger than life man who has it in his destiny to fight the unexpected foes. In order to be the epic hero that he is, he must go i quest of formidable foes.
Beowulf gathers his men and they go by boat to where Hrothgar lives. He learns of the terrible things that have happened to the people. He vows that he will bring Grendel down and he is offered a reward for killing Grendel. However, for Beowulf it is more about the challenge than a reward.