Grendel is presented as a monster, a demon, and a fiend. He was "spawned in that slime," a reference to the biblical story of Cain slaying his brother Abel, a horrible sin. Grendel is the incarnation of evil itself.
Grendel instills horrendous fear in the Danes, and with good reason. He haunts the moors and marshes and attacks the warriors while they sleep at Herot. Grendel is carnivorous; he feasts on human flesh and drinks human blood. He is incredibly strong, once smashing thirty men at one time before dragging them all back to his lair. Despite his size and strength, Grendel can also be quick, quiet and stealthy. He can sneak into Herot without being detected. Grendel hunts at night in the darkness. He has "swift hard claws," "powerful jaws," and "great teeth." Grendel also has magic powers. He has cast a spell so that the warriors' weapons cannot hurt him: "[T]he hardest iron could not scratch at his skin."
Grendel is filled with hatred and blood lust. He terrorizes Herot completely until Beowulf comes to help the Danes. The fact that Grendel is such a dangerous, horrible monster emphasizes the greatness of Beowulf when Beowulf destroys him.
In Beowulf, a poem with a pagan origin but "modernized," most likely, by a Christian monk in what was the kingdom of Mercia sometime in the 8th and 9th centuries, Grendel occupies an interesting position as an enemy of man: on one hand, he is an example of a monster, a force of nature, in a pagan belief system; on the other hand, as the Beowulf poet makes clear, Grendel is a descendant of Cain, the first murderer in the Christian belief system:
With fulsome monsters/this sorrowful man [i. e., Grendel] had stayed awhile,/since the Shaper [God] had condemned him/as Cain's kinsman (ll.104-107).
No matter what belief system the Danes followed, whether pagan or Christian, Grendel represents evil because he is either no longer a part of mankind (from a Christian viewpoint) or he is a force of nature set in opposition to the welfare of mankind (from a pagan belief system). In modern psychological terms, Grendel is "the other."
In the Scandinavian and Anglo-Saxon societies that form the backdrop of Beowulf, disputes among tribes and families could be resolved in only three ways--exile of the guilty party or parties, combat between individuals or tribes, or the payment of money to the aggrieved party (an individual family or a tribe). In the case of Grendel, however, who is not part of mankind, the poet tells us that
Grendel fought/a long time against Hrothgar, hate-feuds he waged,/crimes and murders, for many seasons . . . he wanted no truce/with any man of the Danish forces,/to let them buy peace, pay him a fee. . . . (ll. 151-156)
The Danes are used to an orderly world in which disputes could be settled in some acceptable way (combat is acceptable, but not the first choice), but Grendel's hatred of mankind presents an insurmountable problem: they believe Grendel cannot be defeated by combat, he cannot be exiled (because he is outside the world of men to begin with), and he has no interest in stopping his attacks for money because money is meaningless to a force of nature. The cliche "between a rock and a hard place" sums up the Danes' position.
Grendel is the "perfect storm" of enemies--a being forever cast outside the world of mankind; a force of nature that no Dane has been able to fight successfully; and a being whose hatred of mankind cannot be extinguished, except through Grendel's death. From a Christian viewpoint, Grendel is only one step removed from Satan, and from a pagan perspective, Grendel is part of nature, which is always in conflict with mankind.