The epic tale of Beowulf is a text depicting the fight between good and evil. While the story has changed over the generations it has been told, the final version proves to be one which elevates the importance of Christian thought and power over that of Pagan thought.
In the beginning, Beowulf was a tale told by scops and passed down by word of mouth. Given that the written word was limited, tales were used to entertain and tell of the heroes of the times. Given Beowulf was not written down until many,many years after its beginning, the tale has most likely changed dramatically over the generations.
In the end, the tale has become one of Christian ideology and belief over Pagan belief and ideology. Since Beowulf is given the title of the hero, and the text is from a Christian perspective, he represents the good, or Christian. Given that Grendel, his mother, and the dragon represent Beowulf's epic foes, they all represent the evil, or Pagan.
IN order for Beowulf to find his reward in heaven, he must defeat his enemies (with the help of God). Many times throughout the tale, Beowulf acknowledges that God is the one who will determine the "winner" of every battle. Since Grendel is a God-hater, one can easily align him with darkness and, therefore, a foe of God (and those who fight for God). Essentially, the dragon, Grendel, and his mother symbolize the darkness and evil in a Christian world.