In the Old English poem Beowulf, how does the contrast between Grendel and Beowulf turn their conflict into a fight between good and evil?
Within the first two hundred lines of Beowulf, we learn not only that Grendel was cursed by God "as Cain's kinsman" (l. 107), referring to the treacherous killing of Adam's first son, Abel, by his brother Cain, but also that Grendel is no longer even a part of the human family:
That fiend from Hell,/that grim spirit, was called Grendel,/notorious bush-roamer, at home on the moors,/the fens and fastness. (ll. 101-104)
Even though Grendel was once a man, his condemnation as Cain's kinsman causes his banishment "away from humankind" (l. 110), and he lives with other creatures at war with God--"eotens and elves and orkneys . . . who fought with God." (ll. 112-113). Clearly, Grendel, as a product of evil, is a soldier in the army of mis-begotten creatures who fight against God who, by definition, represents good.
Beowulf, on the other hand, is also as clearly introduced into the poem as a figure of good:
. . . he was of mankind in might the strongest . . . big-hearted and brawny . . . That good man of the Geatish folk/had chosen good fighters. . . . (ll. 194-204)
Just as Grendel is described as cursed and part of the struggle against God, Beowulf enters the struggle against evil as the symbol of all that is good and, more important, powerful enough to effectively fight Grendel. In addition to being good, however, Beowulf also bears the mark of a good leader ("had chosen good fighters"), another very important attribute in the warrior culture of Scandinavia and, later, of the Anglo-Saxon world.
Another very important contrast between Grendel and Beowulf lies in their heritage. Just as Grendel is cursed because he is related to Cain and therefore cast out of the world of men, Beowulf is the son of a great Geat chieftain:
My father was well known to folk,/ a noble chieftan by the name of Ecgtheow . . . any wise man/readily recalls him around the Earth. (ll. 263-266)
In this culture, one's heritage was a critical component of status--if your father was good and powerful, it was assumed, until you did something to the contrary, that you would carry on that positive heritage. The Beowulf poet fully understands the importance of linking Beowulf to his famous father, Ecgtheow, as he earlier had linked Grendel with the murderous son of Adam, Cain.
The differences between Grendel and Beowulf, most importantly established by their vastly different heritages, result in a classic struggle between good and evil, between God and, implicitly, the Devil. The result of this struggle, of course, is dictated by the belief systems of the culture, a culture that was on the cusp of becoming Christian, in which the only appropriate end was the triumph of good over evil--in this case, embodied by a relative of Cain and the son of a powerful and good Great king.