Beowulf, at least the modern translation, is written from a Christian perspective. Therefore, the symbolism of the monsters (Grendel, Grendel's mother, and the dragon) is that of evil. Since Beowulf is good, and on the side of God, that which is against Beowulf is against God.
Another view regarding the monsters is that they symbolize sin. Grendel, and his detest of the light (because he cannot live it it) symbolizes envy: "Then an evil creature who dwelt in darkness, full of envy and anger, was tormented by the hall's jubilant revel day by day." Grendel's mother symbolizes wrath (given her actions are fueled by the murder of her son). Lastly, the dragon symbolizes greed. It cannot fathom that one piece of its treasure is missing. Regardless of the vast treasure, the dragon must have the one piece which was taken.
One final definition of the monster's symbolism revolves around "the evil of human suffering caused by natural disasters" (taken from the eNotes Themes page on Beowulf). While the page denounces the accuracy of this idea, the symbolism behind the monsters in regards to human suffering has been defined previously.