Grendel is your typical monster - living in dark places, alone, having no one but his equally evil mother to communicate with. He is lonely and loveless, and jealous of people who can have all the things he so deeply desires. This is why he became so devious (we don't know for sure whether he was bad from his very birth). Due to his evil nature, he goes about killing people who are allowed to live happy and fulfilled lives, something he cannot ever have. On the other had, we have Beowulf who is a textbook example of a hero. He is strong, fierce, brave, and the only one who can help a king in a dire need of a miracle. He arrives at Heorot in Denmark to aid Hrothgar, the king of the Scyldings (Danes) and kill the monster, which he heroically does. He doesn't use weapons at all, but rather rely on his brute strength to defeat his foe.
Hope this helps a bit. :)
