Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Bitter, Tragicomical
Since the story is from the outcast's point of view, it's no surprise that the narrative oozes cynicism and manic dark humor. And it's not only Grendel's voice that comes across this way. Check out the dragon, who takes on a tone that's a combo of Professors Binns (the boring ghostly professor of history of magic at Hogwarts) and Snape (the bitter, cynical Defense Against the Dark Arts guy):
In a billion billion billion years, everything will have come and gone several times, in various forms. Even I will be gone. A certain man will absurdly kill me. A terrible pity—loss of a remarkable form of life. Conservationists will howl. (70)
Or consider Unferth, the brother-killer whose unconvincing cover of an epic hero leaves us with a bad taste in our mouths. It may be wickedly entertaining to watch Grendel peg Unferth with apples when he's down, but it's hard to get much joy out of character who is more or less "darkness made visible" (97).
A Sense of Relief
There are really only two characters who don't let disappointment, violence, sadness or encounters with extreme horrors affect the beauty of their thoughts and words: the Shaper and Wealtheow. For these two characters, even the bad stuff is lyrical and poignant (though in the Shaper's case, we're not totally sure if this is a 100% positive thing, since a lot of the time, he's pretty much producing propaganda).
As for the rest... well, consider taking a mental health break about halfway through the book. Smell some flowers. Look up at the blue sky. Then plow on to the bitter end.