The moral lesson I'm always struck with in Beowulf is selflessness. Hrothgar has been plagued by the ravages of Grendel for a long time; when Beowulf finally hears of it, he simply gets ready to leave. He gathers some men, tells the King--to whom he owes his allegiance, his time, and his talents--he's going to go. His explanation and his motivation are simple: I can defeat this marauder and rescue Hrothgar's kingdom, so I will. That is the pinnacle of selflessness.
