Grendel symbolizes the sin of man, especially from greed.
Grendel is the monster from below. He was condemned by God because of the dispute between Cain and Abel, when Cain killed Abel. Grendel was the son of Cain.
Grendel was this grim beast called, who haunted the moors and secluded fens; this accursed one had long dwelled with monsters since the Creator had decreed his exile. (ch 1, enotes pdf. p. 7)
Grendel haunts the humans and takes advantage of their weaknesses. When the men enjoy some partying, Grendel comes out as soon as they are off their guard. As long as fear of Grendel keeps the men in check, Grendel stays away. When the partying begins again and the men pass out, Grendel returns.
“AS SOON AS night had come, Grendel set out to explore the lofty abode and to mark how the Ring-Danes had gone to rest within it after their revelry was done” (ch 2, p. 8)
Grendel shows up to exploit the men’s weaknesses. He exists because of God’s revenge on Cain, and he therefore seeks revenge on the greedy humans. Beowulf rescues the kingdom from Grendel, and thus from the consequences of their debauchery.
Beowulf is an epic. An epic hero represents the values of a culture. So it stands to reason that a monster would symbolize its fears. Grendel is the first of three monsters that represents the evils that Anglo-Saxon society feared most. Grendel is described in the opening lines of the poem as a
powerful monster, living down
In the darkness . . .(lines 1-2)
A few lines later we learn that he hates the "harp's rejoicing/Call and the poet's clear songs (lines 5-6), and that he lives alone haunting the moors. The poet immediately associates him with evil, identifying him as a descendant of Cain. But what kind of evil does he truly symbolize? The Anglo-Saxon society was a warrior society. It represented a group of people bound by loyalty to each other, traditions of socializing in the mead-hall, and obedience to the king. Grendel represents the opposite. He is anti-social, does not observe man's laws, and owes his obedience and loyalty to no one. He murders ferociously, ruthlessly, and irrationally. One of the saddest lines involves the Danes' attempts to make a peace treaty with Grendel as they recount
how Grendel's hatred began,
How the monster relished his savage war
On the Danes, keeping the bloody feud
Alive, seeking no peace, offering
No truce, accepting no settlement, no price
In gold or land, and paying the living
For one crime only with another. (lines 64-73)
Here we see that Grendel represents what could be the destruction of Anglo-Saxon society. He is an outsider from the wilderness who observes no civil rules and will not negotiate. He fights dishonorably, killing by ambush and without discipline. As we see later, Beowulf symbolizes the opposite. He is disciplined, social, and loyal. If Beowulf is the hero and savior; Grendel is the enemy and destroyer.