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

As a king, Hrothgar is, in a way, God's representative on Earth. He had even built his hall Heorot in God's honor. Contrary to him, Grendel is a devilish creature, an envoy of Satan himself, and a descendant of Cain, the Biblical figure who killed his own brother. The author wanted to underline this opposition as the ultimate cause of their enmity. The two of them can never reconcile; in order for the community to prosper, Grendel has to be exterminated. That's why Grendel is so bloodthirsty and furious. Also, that's why he can't stand light, as it is a symbol of God's power.