This is actually an interesting question. From a characterization point of view, we don't really get a lot of insight into Grendel. On the other hand, from a plot point of view Grendel is the villain. There has to be someone for our hero to fight, and we'd probably think Beowulf was cruel if he attacked a fluffy defenseless creature as opposed to a man-eating monster!
The text also specifically says that he can't stand the noise of the "revelry" in the Hrothgar's mead hall. Grendel is perhaps attacking because the joyful camaraderie reminds him each day of his own isolation -- owing to his connection to the Biblical Cain.
There is a sense in which we can relate Grendel to a simple dichotomy of good/bad. Grendel at various points in the text is likened to a child of the devil and is described as being wicked and evil. By contrast, humans and in particular Beowulf are described as agents of good. Therefore, this creates an opposition. Grendel, having been shunned by the world and being an agent of the devil therefore will attack those who support the powers of good.
Basically, Grendel attacks humans for one reason. He, a descendant of Cain, has been exiled into darkness. Therefore, as a foe of God, Grendel is angered at the fact he cannot exist in light. Given that he cannot enact his revenge upon God himself, Grendel enacts his revenge upon God's people.