Grendel was the name of this grim demon haunting the marches, marauding round the heath and the desolate fens; he had dwelt for a time in misery among the banished monsters,
Cain 's clan, whom the creator had outlawed and condemned as outcasts. For the killing of Abel the Eternal Lord had exacted a price:
Cain got no good from committing that murder because the Almighty made him anathema and out of the curse of his exile there sprang ogres and elves and evil phantoms and the giants too who strove with God time and again until He gave them their reward. (102-114)
(Heaney)
This quote illustrates why Grendel was evil. It describes him as being a descendant of Cain and banished from the grace of God In addition, there are passages containing expressions in disapproval of heathen worship. There is one of these in the introduction to the Danes near the beginning of the poem.
Sometimes at pagan shrines they vowed offerings to idols, swore oaths that the killer of souls might come to their aid and save the people. That was their way, their heathenish hope; deep in
Bibliography: Heaney, Seamus. Seamus Heaney 's Beowulf: A New Verse Translation. Farrar,Straus, and Giroux, 2000.