The dragon tells Grendel that man needs Grendel, that he is the brute existent by which they learn to define themselves? Is there some truth to this? Does man need opposition to define him? What are some ways that man defines himself by opposition?

Sure, we all need opposition to define ourselves. The is the hero's way to see possibility. Opposition reminds us that our point of view is just and worth fighting for. This has little to do with right and wrong rather than simply finding identity. Opposition defines who we are. Pick a war, any war. Okay I'll pick one. Consider the whole 911 crisis in America. Why was Osama bin Laden plastered on every magazine and newspaper? He was enemy number one. Americans, rightly or wrongly, were supposed to hate him. Osama helped define who Americans felt they were.