In the epic Anglo-Saxon poem Beowulf, there are several motifs. One deals with the supernatural, another with violence and the third with death.
In literature, the supernatural refers to anything that is beyond the realm of nature. This would include God. Unlike our modern-day perceptions of ghosts, poltergeists, etc., the term supernatural in literature refers to anything that is not normally found in nature. In this story, Grendel is but one supernatural being. His mother (Grendel's dam) is another. The description of Grendel and all those related to him present the very essence of the supernatural. (While Shakespeare for the first time [it appears] presented fairies and imps as playful creatures, many people of his era still believed in the very real existence and evil nature of ghosts, witches, etc.—an idea also common to the Anglo-Saxon period.)
In this tale of bravery, monsters and death, the anonymous author refers to things that were cultural norms. The story demonstrates the melding of pagan beliefs with the teachings of Christianity as the Roman Catholic Church sent its representatives to the Britons to convert non-believers.
The monster Grendel is believed to be the descendant of Cain, the first murderer in the Bible. The Anglo-Saxons believed (as shown in the poem) that Cain and all of his descendants were cursed for Cain's murder of his brother Abel.
In Burton Raffel's translation, the reader learns of Grendel's lineage, as well as the creation of all kinds of evil creatures: spirits, giants, goblins, etc. Grendel himself is called "monster," "demon" and "fiend." We also learn of God's punishment delivered to all those descended from Cain.
So Hrothgar’s men lived happy in his hall
Till the monster stirred, that demon, that fiend,
Grendel, who haunted the moors, the wild
Marshes, and made 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. The Almighty drove
Those demons out, and their exile was bitter,
Shut away from men; they split
Into a thousand forms of evil—spirits
And fiends, goblins, monsters, giants,
A brood forever opposing the Lord’s
Will, and again and again defeated. (15-29)
In the eighth chapter, after Beowulf has arrived and offered his services to King Hrothgar to end the twelve-year killing spree Grendel has visited upon the Hall of the Hart (Hrothgar's mead hall), Unferth speaks "quarrelsome words," attacking Beowulf's reputation as a great warrior.
Unferth's challenge to Beowulf—implying that Beowulf has been defeated by his friend Breca—is important because it allows Beowulf to establish his credentials...which enhances his stature in the eyes of the Danes.
Violence was the means by which a warrior proved his valor: something greatly admired by not only the Geats (Beowulf's people), but also the Danes (as noted above). It was not gratuitous violence, but proving one's mettle in warranted battle—honorably and bravely. Unferth's words are born of his jealousy of all Beowulf has accomplished. Beowulf answers the challenge with his side of the story to which Unferth has referred. He states the fact that no one had battled more nobly in water—in his fight with Breca—than he. He does not brag, for that would be a sign of false pride. For the Anglo-Saxons, Beowulf is the ideal hero.
Death is another major motif in Beowulf. It is not something that the warrior Beowulf fears: for him, fighting is an honorable act; attaining glory in brave battle is to be desired; and, dying in the midst of such a noble pursuit is perceived with stoicism, as shown in Chapter 22:
...with Hrunting [the sword] I now seek glory for myself, or death shall take me.
Most of the men that Beowulf brings with him are resigned to the very real possibility that they shall not survive the night against Grendel's attack. What sets Beowulf apart, however, is that he is resigned to this potential fate. His attitude demonstrates what a strong and brave character he is. The scop (storyteller) that would have regaled his audiences with the tale of mighty Beowulf would have been offering up praises of a man unlike most men: a hero that is brave, cares for the well-being of others, handles himself with honor (his word is his bond), and is prepared for the destiny (as the poem puts it) God has chosen for him.