Part of the answer to this question lies in the allegorical nature of the text and the way that Beowulf presents a titanic struggle between the forces of good and evil, with evil of course being represented in the form of Grendel. Throughout the text, every reference to Grendel uses adjectives such as "fiend in hell" and "ghastly atrocities." Such descriptions clearly associate him with demonic, evil forces that are opposed to the good that is represented in the order that Hrothgar tries to sustain and Beowulf defends. Consider the way that Grendel is presented in the following description:
...misery of men; that damned creature,
grim and greedy, soon was ready,
savage and cruel, and from their rest seized
thirty thanes; thence back he went
pround in plunder to his home, faring
with banquet of bodies.
The text does not give any indication of a specific motivation that lies behind Grendel's attacks: there is no reference, for example, to a particular grudge that Grendel has against Hrothgar. It is enough that Hereot exists and that Hrothgar is there with men. Grendel's status as a "damned creature" who is both "grim and greedy" and "savage and cruel" indicates that his motivation for murder stems from his evil nature. As the personification of evil, he is bitterly opposed to the goodness presented in Hrothgar and the men at Hereot. As the force of evil, he is implacably opposed to any force of good and seeks to destroy Hrothgar's kingdom.