John Gardner's novel Grendel begins in the twelfth year of Grendel's terrorizing of Hrothgar and Heorot, and perhaps the best phrase to describe his tone is defiant rage. 

Grendel is mad at everything: the time of year, an old ram standing on a cliff, the weather, and particularly the fact that he is beginning the twelfth year of his "idiotic war. The pain of it! The stupidity!" One of his first actions in the novel best expresses his rage. Grendel says:

"I let out a howl so unspeakable that the water at my feet turns to sudden ice and even I myself am left uneasy."

It is clear that he is disgruntled and outraged about everything that is happening in his life and around him at this moment, not the least of which is his ongoing but ridiculous feud with Hrothgar. 

Oddly enough, he displays little of this pent-up anger when he heads to Heorot to perform his usual bloody, savage raid on Hrothgar and the the rest of the village. Instead he rather calmly takes his conquests and eats them with great delight. In the morning, however, Grendel's rage surfaces again as he hears the mourners singing at the funerals of those who were killed; in his mind, the songs somehow seem to be taunting him.

This tone of defiant rage clearly reflects Grendel's dissatisfaction with his life in the opening chapter of this novel. Unfortunately for Grendel, things are not going to get much better for him.