In his search for meaning, it seems that Grendel has come to define himself as antagonist to humans. Is this meaningful? Is he defining himself here, or is he reacting to his surroundings and allowing those surroundings to define him?

That's getting pretty deep. Would you be upset if I said both? Really Grendel is a product of his own demonic nature. Hey, he is a demon and he likes to kill things. At the same time Grendel is an introspective monster. He muses over the Danes and the words of the shaper, he begins to define himself by them.