What drives Grendel to attack so many men at Heorot, the mead hall?

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.