In the story of Beowulf, the theme of good vs evil is introduced in Section 1. Descended of one known as Beowulf Scylding, Hrothgar becomes a king of the Danes, and builds a mead hall, where men join to visit and celebrate. The sound of "revelry" (rejoicing) angers Grendel, a human-monster that lives in the fens (swamps).
Section 1 tells of Grendel's heritage: he is a descendant of Cain (son of Adam and Eve), who murdered his brother Abel, in the book of Genesis in the Bible. Because of Cain's sin, he is exiled, and Grendel, his descendant, is also exiled from the company of men.
On kin of Cain was the killing avenged
by sovran God for slaughtered Abel.
Ill fared his feud, and far was he driven,
for the slaughter's sake, from sight of men.
Of Cain awoke all that woful breed,
Etins and elves and evil-spirits,
as well as the giants that warred with God
weary while: but their wage was paid them!
Grendel is considered evil, as was Cain (who sired his line), elves, evil spirits, and giants. None of these is at peace with God.
And so the greater conflict here, beyond simply good vs. evil, and Grendel vs. man, is God vs. all those creatures who have "warred" with him. This could be translated to mean God vs. the Devil, or the powers of darkness.