This question is surprisingly difficult to answer, for three reasons:
- Beowulf is an old tale, surviving in a lone manuscript, written after (presumably) a long life as an oral tradition, long before there was such a thing as “psychological motivation” in literature;
- Grendel is a beast of the land, living in a pool with his mother; the only “reasons” for his actions are animal motivations: hunger, protection of his territory, fear of outsiders, pre-emptive strike, etc.;
- Grendel is a fictional creation, not a real creature, so the important question is: What is Grendel’s function in the story; that is, how does he function as a literary invention? What did the “author” want to use him for?
It is clear that the Danes are intruders in Grendel’s environment. It is also clear that the author of the tale wanted Beowulf’s actions to be “heroic”, that is, super-human, beyond the usual acts of bravery by any soldier, so the “foe” had to be super-strengthed, not just a mere mortal enemy. It was also important that the “foe” was symbolic of all fears of Man, especially the fears of Nature’s as yet undiscovered dangers (a parallel is found in legends of sea creatures beyond normal sea-lanes). Grendel does this fictive task well, by appearing “in their sleep” or during their drunken stupors, and by mutilating his victims in an animal-like way. More naturalistically, Grendel saw the Danes as intruders, competitors for his territory and game, and a foreign enemy.