In "Beowulf" there is no sympathy nor motivation for the monster's actions. Grendel is a flat (or static) character rather than a round one. He is simply an evil that the intrepid Beowulf must dispose of. There is no compassion at all for the beast, only rejoicing after Beowulf slays Grendel and hangs his arm and claw from the rafters of Heorot Hall.
In Gardner's take on the epic, Grendel is a round character and the story of Beowulf is told from his point-of-view. Gardner carefully establishes a history for his protagonist. Grendel is a descendant of the doomed lineage of the biblical Cain. Gardner also establishes a relationship between King Hrothgar and Grendel. The monster tries to make friends, but is not successful. Finally, and perhaps most poignantly, we see the "monster" at home, where his mother is unable to communicate with him. Grendel is completely alone and lonely. Who wouldn't eat a few people? Is that so wrong? :)