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.