portrayed as someone with a heart and sense of humor. We are even given gut-wrenching quotes that show Grendel is a character with huge inner turmoil.

Why can't I have someone to talk to?

Gardner even gives readers the idea that Grendel is in a position not by his own choosing. Grendel is the unfortunate side effect of some long lost descendant; therefore, Grendel is living a curse. That curse causes him pain, suffering, isolation, and loneliness.

He told of an ancient feud between two brothers which split all the world between darkness and light. And I, Grendel, was the dark side, he said in effect. The terrible race God cursed.

Deep down, we know that Grendel is doing bad things; however, we also know that he can't always help it. The actions he is taking are still brave. He's stepping into battle with the threat of death. That's a standard hero affair.

In short, Grendel, as he is portrayed in John Gardner's Grendel, is a point of view that subverts the status quo. 

Grendel reveals that humans fool themselves by rewriting history, by applying meaning where meaning doesn't exist (in the signs of the zodiac, for instance), by creating gods where they don't exist.  Humans constantly lie to themselves in order to emotionally and mentally survive and continue to live.

Grendel is the teacher of reality in the novel.  He is the bringer of truth.  He is the destroyer of illusion.  He thinks about killing the queen in order to teach the humans reality.  He refuses to kill Unferth and instead carries him back to the mead hall, in order to destroy the myth of the great, superhero-like hero.   

Grendel points out the foibles and shortcomings of human society.  He is a rebel and an outsider. 

These make him an anti-hero.

At the same time, you should remember, he is also an unreliable narrator and he, himself, takes on the role of his nemesis, the Shaper.  Grendel becomes an artist, a rewriter, if you will.  He writes the novel.  He tries his hand at poetry.  He embraces art, even though he despises it.  He both loves and hates art.  And though Unferth is his goat, so to speak, he also reveals a kind of nobility--a kind of heroism--in Unferth.  Humiliated, embarrassed, sinful (killed his brother), Unferth is still the warrior who sleeps in front of the queen's door to protect her. 

Maybe humans do lie to themselves in order to survive.  But they do survive and endure.  The novel is about reality vs. art.  And art wins.

John Gardner presents Grendel as a complicated, imperfect character who shows many of the characteristics of a hero. Rather than the simplistic, brutal monster of the traditional Beowulf epic, Gardner’s Grendel is creative and thoughtful. One of the main similarities with classical heroes is that Grendel embarks on a hero’s quest. He searches for meaning, however, rather than an object. In a world that is not easy to understand, he cannot conclude that he has found meaning.

Grendel is often—but not always—courageous and is not easily deterred. Both in ferocity and stamina, Grendel follows the way of the warrior. He maintains the courage of his convictions, which requires him to oppose the much more numerous forces of the Danes. When his overtures of peace are rebuffed, he pursues vengeance against those who wronged him.

Grendel is a solitary hero who fights on, even when the odds are against him. Rather than a leader of an army, he is a loner but not necessarily by choice—he is apparently the last in his line. It seems that he and his mother are the only surviving members. Although he understands that their lineage labors under a curse, he strives to gain release from its restrictions.

John Gardner's Grendel is written from the monster's point of view, or more precisely, the work is presented as if the monster wrote it.  First of all, then, readers are likely to identify with the point of view any story is told in.  That's a basic fundamental of story telling.  Thus, we at least somewhat identify with Grendel.

Other than that, however, the answer to your question is extremely complex, and fun in the way that the book is fun. 

Grendel, as narrator, shows himself playing mind games to entertain himself, and literary games, if you will, as he writes the novel.  And the reader sees Grendel grow as a writer/artist as the novel progresses.  He writes a chapter as if it were a screenplay.  He writes poetry, beginning with weak "doggerel" poetry, ending with some pretty good stuff, and experimenting with iambic tetrameter and other rythms and meters along the way.  He connects each of the twelve chapters with a corresponding astrological sign.  He says that when he was younger, when he was still playing cat and mouse with the universe, he used to play games.  But of course, he still does.  He is not a nihilist as the dragon is--he still plays games to entertain himself, and is still hoping for some kind of meaning in existence.

Perhaps most specifically, to answer your question, Grendel, you should realize, is a quintessential unreliable narrator.  Even though he ridicules humans for rewriting history and creating art and for attributing meaning where there is none (astrology itself, the rewriting of history for the sake of art, the gods, etc.), that is exactly what he does.  In writing the novel, Grendel is imitating the Shaper. 

Grendel is exposed to and relates various philosophies throughout the novel:  nihilism, political anarchy, existentialism, etc.  Grendel, espousing existentialism throughout much of the novel, agrees with the dragon (the nihilist) that there is no pattern to the universe and existence is inherently meaningless.  Unlike the dragon, however, he doesn't just sit on gold.  He finds meaning, at least of a sort, the same way the humans do--through art. 

His pounding of Unferth with apples is art, of a kind:  it's also slapstick comedy.  He dances and tips an imaginary hat and sees the humor in and thinks it's funny when the humans cover the light because he, accustomed to living in an underwater cave, sees in the dark. 

And he writes.  Grendel's universe is a universe of mere chance, with no inherent meaning.  Beowulf could have lost.  But Beowulf wins, only by chance, because Grendel slips.  That is Grendel's interpretation, predicated on his philosophy. 

Is Grendel a hero?  Only in the sense that anyone who intellectually sees the universe as mere chance, but still wakes up in the morning trying to accomplish something, is a hero.  But in the end art is the hero of the novel.  It is art that makes living worth while.

Grendel is not a hero in any traditional sense, however, even though we tend to identify with him instead of the humans, who are at least somewhat presented by him as idiots.  He eats children, at least partially, to make the heinous point that they should have listened to their mother.  He eats children as a gruesome joke.