What is the purpose of referring hrothgar and grendel in terms of their relationship to god

Grendel a vicious creature who saw the world only in two ways, those things which can be killed and those who come in the way of killing those things, this represent the Devilish nature of Grendel, who lineage is traced back to Cain the one who killed his own brother according to Bible. On the other hand Hrothgar is a king, God's successor and representative on Earth. These two are the personification of the eternal war between Satan and God i. e. darkness and light.