Question

What do the details about grendel's origins and dwelling place add to your impression of him?

Answer 1

Grendel is living in a place we can refer to as hell (metaphorical), the place is inside mountains, a kind of cave with intense smell and waste, which produces an impression of Grendel as a vicious and insensitive entity. 
As the origin of Gredel is tracked back to Cain who killed Abel that is also adding up to the description of him being vicious, selfish and insensitive monster.