What descriptions associate Grendel with death or darkness, and how are they meant to make us feel about 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...".