Answer:
He is feared by humans for his frightful visage and likely grew to despise humans for their treatment of him. He is also bloodthirsty.
Step-by-step explanation:
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.