Grendel follows the philosophical evolution, from solipsism to nihilism, of a socially isolated creature, a monster. It is an examination of human metaphysical curiosity and its many dangers, specifically the tendency toward blind cynicism. Grendel is a censure of the rapid growth of this cynicism in twentieth century society and the consequent widespread distrust of abstract ideals.
In investigating his own nature, the monster in the story destroys himself. He realizes that the universe is determined, accidental, and so he loses faith in his own importance. With time, he becomes a beast, a "brute existent," until eventually his soul has wholly left him. He does not die for love or for passion or for freedom. His spirit dies instead simply, hopelessly, mired in boredom and indignation, without courage or sadness. Grendel is dead long before his body fails him. He fades away, and the most important theme in this novel is that such self-destruction, though tempting, is not the only answer.
Grendel's first defense against a brute universe is solipsism: the belief that the self can know only itself and that it is the only existent thing. As the novel clearly demonstrates, solipsism is a weak defense. However loudly a creature may declare its godhood, the universe continues to function independently. However staunchly a creature may deny the reality of outside factors, outside factors will continue to impose themselves upon his everyday existence. The main flaw in solipsism is that it contradicts every aspect of human experience. Grendel is justifiably unsatisfied and nervous under its tenuous cover.
After his visit to the dragon, Grendel finds a more solid shield, nihilism: the belief that nothing has meaning; that life is a long series of accidents and is in itself an accident. As a philosophy, nihilism is complete. It provides an answer to every metaphysical, "why?" Whatever the concern, a nihilist can conclusively state, "There is no way to truly know, but it does not matter anyway.