As other contributors have already pointed out, Grendel is acting out of envy and spite. Miserable, he lashes out at the Danes, jealous of the happiness which human beings can enjoy. In this sense, his attacks against Heorot reflect a deeply personal slight not only against the Danes, but also against the entire human condition, one which (I suspect) ties sharply to the poem's underlying Christian themes.
Remember, the universe of Beowulf is a Christian one, and Grendel's own background ties directly back to Judeo-Christian origins, given that Grendel himself is explicitly presented as a descendant of Cain. When reading this poem in a theological lens, Grendel actually appears to be one of the most fascinating characters in the entire saga, with his grudge against humanity, in particular, reflecting central concerns of the Christian universe.
Critical to this analysis is the concept of sin—more specifically, sin within the Augustinian mindset. Sin, as Augustine understood it, was a state of being, one built into the human condition and the entire natural order. Grendel, I would suggest, almost seems to embody this particular vision of original sin. For one thing, consider how his evil nature is described as being transmitted directly down to him through his descent from Cain (a descent which parallels the Augustinian notion of original sin, which itself traces this state of corruption back to Adam himself).
He is a creature entirely defined by his own malice: he is cruel, vicious, and sadistic. His evil is so all-encompassing that it defines his very nature and, loathing the humans that surround him (humans who moreover enjoy positive virtues and qualities that he can never possess), he attacks the Mead-Hall, looking to create as much suffering and misery as he can inflict.