The mood of Grendel's story, as told in his own first-person narration, shows a suprising depth of feeling when compared to the depiction of the monster in the original Beowulf poem. Grendel feels, loves, hates, jokes, and, often, suffers. That Grendel has moods at all is Gardner's profound innovation from the thoughtless, murderous beast in Beowulf.
The predominant mood throughout Grendel's story is sadness. Grendel feels that the world is unfair and unjust, especially toward him. The natural world, silent and unyielding, forces him to become a lonely outcast due to his uncontrollable killing that disrupts the normal progression of life and death. This feeling creates the novel's mood of tragic unfairness, from Grendel's perspective. Similarly, Grendel expresses righteous outrage at the hypocrisy of mankind, their reckless warmongering and gullibility for the Shaper's stories. These feelings show Grendel's moral fortitude tinged with disdain, complicating the self-deprecating mood with which Grendel describes his own situation.
Despite all this, Grendel still viscerally enjoys murder. In the passages in which Grendel raids the castle, especially in Chapter 6, the monster kills with emotionless ease. In the moment, to Grendel, murder feels as normal and natural as eating when one is hungry or sleeping when one is tired. The mood in these passages is calm and assured, contrasting with the overthinking self-pity in other parts of the novel. This change in mood foregrounds Grendel's mental journey as he struggles to balance his need for killing with his displeasure at his ostracized position in the natural world.