One of the devices the Beowulf poet uses are kennings, or compound phrases that function as a noun and further re-name a person or place by describing them in an interesting way. One use of a kenning is to help enhance characterization. For example, throughout the poem, "hell-forged hands" is used to describe Grendel. The imagery suggests a creature who was created in hell, which clearly connects to death and darkness. Grendel is also referrred to by the kenning, "Almighty's Enemy", another reference to his evil nature.
It is significant that Grendel attacks at night because he is described as being evil and the darkness symbolizes that evil. Grendel is described as being the spawn of Cain (the Biblical character who was the first murderer). Also, Grendel's nighttime attacks make him seem more sinister because he attacks while the warriors are sleeping - they are helpless at the time. He is called the monster of evil, he leaps and laughs after killing thirty men, and he continues to come back night after night.
The following lines from section II have more language that supports Grendel's link to death and evil.
The monster of evil fiercely did harass,
The ill-planning death-shade, both elder and younger,
Trapping and tricking them. He trod every night then
The mist-covered moor-fens; men do not know where
Witches and wizards wander and ramble.
So the foe of mankind many of evils
Grievous injuries, often accomplished,
Horrible hermit; Heort he frequented,
Gem-bedecked palace, when night-shades had fallen