Why might Gardner have introduced Hrothulf (Hrothgar's nephew) into the narrative? What does this character allow Gardner to explore or suggest?

Hrothgar’s nephew, Hrothulf betrays his cousin and usurps his throne. I think he exists as a juxtaposition to Beowulf's code of loyalty and honour. Gardner is able to explore the flip side of a righteous warrior leader. He represents everything that Beowulf is not.