Duty

I think that duty is something that is self-administered by obligation, fear, inspiration, or merely choice. The goat climbs the because he sees nothing that he is fit to do nothing else. The people listen to the priest and do dull work for their kingdom because they fear the unknown existence outside the established order, they would rather live in boredom than die with excitement. As for the dragon, he is an all powerful being with the ability do whatever he wants, so he chooses to waste on useless riches and arrogance rather than doing something worthwhile because reality is not good enough for him. Grendel, born in an underground pit of darkness by a mindless mother, has nothing to drive him in the real world, nothing to fear, no obligations, nobody to inspire him. Since he has nobody that cares for him in the world, Grendel's duty is torn between two options: to live in isolation forever beneath the earth where the haters won't bother him, or perform the only action he can in a world that will not accept him, terrorize it. Grendel feels that his duty is to terrorize the real world for the sole reason that the world will give him no other option. Duty is a choice driven by the stakes of the world around you, and Grendel's world sees him fit for no duty other than being the villain.