Grendel serves as a multi-faceted symbol in Beowulf, one that mirrors the complex culture and religious background of the poem. One one hand, Beowulf is a poem about a Scandinavian folk hero vanquishing monsters, and on the other, the poem comprises a detailed history of dynastic struggles among the Swedes and Danes, and, perhaps more important, explores the nature of loyalty, war, and kingship. Overlaying the poem are two competing belief systems—paganism and Christianity—that create a constant tension between imagery and world view.
When Grendel first appears, he is described as
That fiend from Hell,/That grim spirit, was called Grendel,/ ... this sorrowful man had stayed awhile,/Since the Shaper had condemned him/as Cain's kinsman. (ll. 101-107)
Although many readers see Grendel as just a monster, the Beowulf poet carefully places him within a Christian context and thereby creates a Christian symbol. As a descendant of Cain, the world's first murderer according to biblical tradition, Grendel has been cast out by God just as God cast out Cain for having murdered his brother, Abel. Grendel's lineage, then, is human—he may be inhuman now, having lived with "eotens, elves, and orkneys," but the poet implies that Grendel is a debased human, not another species.
Because Grendel is also a creature related to pagan enemies of mankind in the Scandinavian belief system also present in Beowulf, he becomes the symbol of nature as predator. Despite his origins within the human family, Grendel eats men, the ultimate sign that he has become, through his isolation and living with monsters rather than men, something other than man. In the Scandinavian pagan belief system, nature is more often seen as the enemy of mankind than as benign or even neutral. The Old English poems "The Seafarer" and "The Wanderer" clearly express the dangers and hardships brought upon mankind by nature, and Grendel is a part of that natural danger.
To further distance Grendel from mankind, the poet points out that he not only attacks Hrothgar and his men constantly without cause, but Grendel also ignores cultural norms, further evidence that he is not part of mankind or the society he attacks:
... hate feuds/crimes and murders ... he wanted no truce/with any man of the Danish forces,/to let them buy peace, pay him wergild (man-money, compensation) ... (ll. 152-56)
Grendel, being a hater of men, does not accept traditional ways of stopping constant warfare. In Scandinavian culture, war can be avoided if the weaker party is willing to pay a price, and the murder of a man, rather than resulting in a war between tribes, can be solved by the murderer paying compensation to the murdered person's family and tribe. In the eyes of Hrothgar and his people, Grendel becomes the symbol of implacable hatred, something that will not respond to reasonable ways of solving conflicts.
Lastly, the Beowulf poet places Grendel in the realm of magic, a symbol of supernatural strength and invulnerability. When Grendel attacks the Geats in Heorot, where Beowulf awaits him unarmed, the poet notes that the Geat warriors tried to attack him with their weapons, but
... no war-blades,/the choicest of weapons anywhere on Earth,/could even scathe that sinful wrecker; for he'd cast spells against all/edged weapons whatever. (ll. 802-06)
The advantage that Grendel gives himself by magic, however, is short-lived. Beowulf, not wanting to take advantage of an unarmed man (being), is not carrying a weapon and, in the event, Beowulf grapples with Grendel and tears his arm off at the shoulder, a fatal wound.