The monsters in "Beowulf" are all broadly symbolic of the marginal outsider in society, something to be isolated and destroyed to maintain social order. However, each monster has its own specific significance, whether drawing on biblical or mythological symbolism.
First, it is argued that Grendel's mother is a reference to the bible character of Cain, the son of Adam and Eve who murdered his brother Abel. Grendel, meanwhile, is thought to represent evil from the standpoint of Scandinavian mythology, meaning things like general social disorder, such as theft and killing for its own sake. The third of the three monsters in "Beowulf" is the dragon. This mythical creature has long been emblematic of sin, the slayer of which is therefore virtuous by definition.