In the Old English poem Beowulf, what leads Grendel to attack Hrothgar's hall, Heorot?
In order to understand Grendel's motivation for his attack on Heorot, we need to examine Grendel's origins. For example, within the first 120 lines of the poem, we learn
That fiend from Hell,/that grim spirit, was called Grendel. . . . With fulsome monsters/this sorrowful man had stayed awhile,/since the Shaper [God] had condemned him/as Cain's kinsman. (ll. 101-107)
Because Grendel is related to Adam's son, Cain, who killed his brother Abel and was therefore cursed by God, Grendel was also banished "away from humankind" (l.110). At the same time, the poet tells us that "misbegotten creatures came to life then," and Grendel, described as "this sorrowful man," spent his time among these monsters and, over time, became one of them.
The key to Grendel's nature, as well as his behavior, is that he was once a man who has been banished from the world of men and has lived with monsters so long--who are at war with God--that Grendel too becomes a monster at war with mankind. We see this in his approach to Heorot when Hrothgar's men, "the Ring-Danes after the beer-party," went to sleep:
The sinful creature,/grim and greedy, was instantly ready,/savage and spiteful. . . . (ll.120-122)
In other words, Grendel's attack on Heorot is simply an act of spite. He attacks men because he knows that he can no longer share in the camaraderie and celebrations of mankind. And this spite is all the stronger because Grendel knows that he was once part of mankind and is forever shut out of that fellowship.