What question does grendel grapple with throughout gardner's novel?

Want to know reasons for his existence.

Explanation:

It should be understood that the novel is about a buerwolf known as Grendel.

And this novel, Grendel is both more animal (he is a solitary hunter) and more humane (he is disgusted with the way the men treat their environment.) And that is why, Grendel tries to fight the ultimate form of humanity, by screaming like an animal.