What drives Grendel to attack so many men at Heorot, the mead hall?
Grendel is jealous of the people celebrating in Heorot. The warriors sing and make merry in the mead hall until one night Grendel decides to attack. Under the cover of darkness, while the warriors are asleep in a drunken stupor, Grendel attacks, and the monster takes away thirty men. The monster’s aim is to feed on the bodies.
He attacks again on the next night and every subsequent night until the mead hall is abandoned. Grendel is driven by evil, and he seeks to disrupt the Danes' way of life and cause them lasting torment. Grendel ensures that the feud lasts for a long time, and all attempts by the Danes to fight or appease him are met with increasing hostility. The monster is only interested in bringing death to the people.
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.
Grendel is frequently described as one of the descendants of Cain, and thus he is forced to wander the world in isolation, cut off from civilization and community. It's suggested that the light-filled joy of Heorot, with its many feasts and gatherings of large groups of people, anger Grendel, and so he attacks the mead hall in order to put an end to the merriment. This aspect of Beowulf is one of the most interesting parts of the poem. Grendel, for all his demonic cunning, is not necessarily pure evil; rather, he's an outcast, an isolated loner who seems to long for some kind of community and is unable to deal with the fact that others experience joy while he does not. Thus, even though he is one of the villains of the story, Grendel becomes almost sympathetic, as he attacks Heorot in order to put a stop to the joyful celebrations that remind him that he is alone.