One of the descriptions of Grendel that most closely identifies him with death and darkness is the description that notes the monster's aversion to the light and happiness of Heorot. The poet very clearly says that Grendel hates the merry celebrations that often take place at the prosperous mead hall, and this fact ultimately leads the monster to begin attacking Hrothgar's people. Thus, the anecdote describing Grendel's aversion to light and happiness also aligns him with darkness and, later on, death.
This description is also interesting because it prompts many readers to feel at least a little sorry for the monster. As a descendent of Cain, Grendel is condemned to wander the earth as a lonely outcast. As such, much of his anger at Heorot and Hrothgar stems from his own loneliness and lack of community. Thus, though Grendel is clearly a villain in the poem, his jealousy in regards to Heorot's light-filled happiness should also make us feel some kind of pity for him.
The first time Grendel is described early in Beowulf, he is described as "...living in darkness,...". A little further on we learn that Grendel lives in a hell on earth and that he was a descendant of Cain. Also, we are told that he attacks at night when it is dark, which indicates that he is at home in the dark more than he is in the daylight. He is also so powerful and evil that he snatches up 30 men at a time to smash them in their beds and run out of the hall with their bodies. He does this for 12 years before Beowulf comes to the land. All of that makes Grendel a clearly evil figure and that is how the reader, or listener, is supposed to feel about him. Later that attitude is confirmed when Grendel attacks for the last time. We are told that, "Out in the darkness the monster began to walk...".