Then his rage boiled over, he ripped open
the mouth of the building, maddening for blood,
pacing the length of the patterned floor
with his loathsome tread, while a baleful light,
flame more than light, flared from his eyes.
He saw many men in the mansion, sleeping,
a ranked company of kinsmen and warriors
quartered together. And his glee was demonic,
picturing the mayhem: before morning
he would rip life from limb and devour them,
feed on their flesh. (723-733)
Grendel actually takes pleasure in the details of his murderous assaults on the Danes, suggesting that he values battle for its own sake, rather than for the glory or the gold that he can get as a result of taking part in it. By contrast, heroes like Beowulf fight for honor and for rewards, not for the thrill of killing.