The epic poem Beowulf is about the life and exploits of a renown Geatish prince named Beowulf. The book Grendel is a description of the life of Grendel, one of the monsters Beowulf kills in the poem Beowulf. A major difference in the minds of these two characters is how and why they act the way they do. Beowulf acts as he does because he sees himself and humanity as always being in the right. Grendel, on the other hand, acts as he does for the enjoyment it gives him and because he sees no reason to stop.
Beowulf is the type of character that acts selflessly. He acts the way he does because he believes it to be in the best interest of the people. In the poem, when Beowulf is leaving Geatland, it says that although he is the most powerful of the king's thanes no one tries to stop him from going. This shows the Geatish peoples jealousy, and perhaps dislike, of him because later in the poem, when he returns, Hygelac tells Beowulf "I dreaded the outcome of your expedition and pleaded with you long and hard to leave the killer be." Beowulf acts the way he does because he knows it is what is right. He doesn't go to Daneland to gain more riches or glory for his name, but to help an ally of his people. Also, he doesn't fight Grendel without sword, shield, or armor to elevate his name should he win, but because he knows it will serve no purpose against the beast. Beowulf's actions show him to be a selfless person who decides how he will act based on its impact on the people.
Grendel's actions, on the other hand, show him to be a person who is only concerned with himself. Grendel does what he does simply because he can, no other reason. His actions portray a person who knows he shouldn't do what he does but sees no reason to stop. In Grendel's conversation with the dragon he is told that he is what defines man. The dragon says, "You are, so to speak, the brute existent by which they learn to define themselves.