Grendel comes to Heorot to kill some of the people there. His motivation to do so goes beyond mere mindless bloodlust, however. Grendel is presented as the ultimate outsider. He is described as a descendant of Cain, a figure from the book of Genesis. In Jewish and Christian traditions, Cain is the eldest son of Adam and Eve, as well as the first murderer. As punishment, God makes Cain go into exile. Being a descendant of Cain, Grendel lives up to his ancestor's reputation: he is a murderer and lives in exile from human community. However, unlike Cain, Grendel has a slightly more sympathetic side, since the narrator hints that Grendel wishes his lot in life were different. In general, the monsters in Beowulf are social outsiders. While Grendel's mother is motivated by revenge and the dragon is greedy, Grendel is envious.
When he hears the music and revelry within the mead hall, Grendel experiences both pain, since he feels so lonely, and envious rage because he is unable to share in the communal comfort the mead hall provides everyone else. Being born from monsters, he can never be among people. As a result, Grendel lashes out at the people in Heorot, slaughtering warriors as they sleep.