Symbolism, Imagery, Allegory
Gardner freely pilfers material from Beowulf. (Since he's openly riffing off of the poem, it's all good, really.) One of the bigger things that makes its way into the novel is the meadhall and all its glorious and tormenting significance. In the novel—as in Beowulf—the meadhall stands for Hrothgar's ability to muster the strength of men from the surrounding areas and for his wisdom in planning and strategy.
With the tribute he gets from neighboring kings, Hrothgar can buy the loyalty of his thanes and concentrate power in his hands. Grendel watches all of this with a kind of fascinated horror. He admires Hrothgar's abilities as a thinking animal, but he feels anger at the wastefulness and brutality of humanity—and sees Hart as symbolic of all that.
Beyond the Pale
But the meadhall represents something even deeper and more complex. The Beowulf-poet makes it clear that Grendel attacks Hrothgar and his beloved Hart because Grendel could never participate in the gift-giving and merrymaking that went on there. Because he is cursed by God, he can't participate in human society. It's not just that the humans are too close-minded to let him into their society; he's also fated not to be let in.
Grendel's exclusion from Club Hart is one of his earliest clues that he is a pawn in some greater cosmic game. And it seems like there's nothing he can do about it except play the crummy hand he's been dealt. Do you think he has a choice? Could he choose not to attack the humans? Why is anyone kept out of the meadhall to begin with?