In the epic poem Beowulf, Beowulf is the Geatish hero of the story, while Grendel is humankind's archenemy.
Beowulf, the story's protagonist, arrives to assist Hrothgar, King of the Danes, defeat the ferocious and murderous monster Grendel, who raids the mead hall until it sits empty for a very long time. Our hero arrives with a long list of victories in battle to his credit. He is a man with unshakable valor and integrity. He is self-sacrificing. His actions are guided (as are other heroes of the time) for glory and for God. Greeted by Hrothgar's wife, Beowulf explains what motivated him to come to the Danes' aid:
This was my thought, when my thanes and I
bent to the ocean and entered our boat,
that I would work the will of your people
fully, or fighting fall in death,
in fiend’s gripe fast. I am firm to do
an earl’s brave deed, or end the days
of this life of mine in the mead-hall here.
On the other hand, Grendel is a savage creature without honor or compassion for any human:
The monster of evil [Grendel]
Greedy and cruel tarried but little,
Fell and frantic, and forced from their slumbers
Thirty of thanemen; thence he departed
Leaping and laughing, his lair to return to,
With surfeit of slaughter sallying homeward.
The narrator reports that when Cain murdered his brother, Abel, God exiled him. Grendel is a descendant of Cain. As Cain was before him, Grendel is isolated from God and the friendship of others. Consequently, the creature finds the sounds of joy and camaraderie coming from Heorot loathsome, which causes him to begin his attacks.
While Beowulf is a brave, proven warrior, Grendel is a creature that fights for the pleasure of killing. Beowulf answers a higher calling in life, but Grendel is answerable to no one. He delights not in glory, but in devastation and suffering.