What is the symbolic significance of Grendel and his mother in Beowulf?

The epic poem Beowulf has an
interesting trait, and it is that it mixes elements of paganism and Christianity. when
it comes to the Christian elements, we find that Grendel, the dragon, and his mother are
described as descendants of Cain. In Christianity, Cain and Abel are recognized as the
sons of Adam and Eve. Presumably, Abel was the good son, while Cain was the bad one. 
According to the biblical story,  Cain killed Abel and is known in the faith as the "kin
killer".

It is also accepted in Christianity that all the
descendants of Cain would be monsters, and other destructive beasts that represent the
sins of the human race. Therefore, the fact that Grendel and his mother are described as
descendants of Cain implies that they represent the forces of evil. Contrarily, Beowulf
and his people are representative of the forces of all that is good, because both God
and "the fates"  gave him a mission to fulfil. The fact that he fulfilled this destiny
and defeated Grendel and his mother (evil) is what gave him his quality of
superhero.