Like many older religious belief systems, the odd amalgamation of pagan and Christian beliefs shown in Beowulf are heavily influenced by the notion of "fate," referring to a Deterministic worldview. In this system, all things are known and fated to occur; people live out their roles in life and die at their appointed time. Grendel, descended from Cain, is destined for a life of hatred and misery; he acts as a force of evil, and has little choice either in his role or in his eventual death. Two examples are shown below:
Ere morning came, since fate had allowed him
The prospect of plenty. Providence willed not
To permit him any more of men under heaven
To eat in the night-time.
The end of his earthly existence was nearing,
His life-days' limits.
(Hall, Beowulf, gutenberg.org)
It is clear that Grendel, like all others, has a specific destiny: he is to kill until he is killed, and his life serves that purpose alone. Determinism tends to be rigid, and not allow for the possibility of changed outcomes. It is therefore almost impossible for Grendel to have acted in any other way, and equally impossible for him to survive the battle with Beowulf. Instead, Grendel was destined from his birth to be a creature of evil; he would be unable to change this destiny, and so there cannot be empathy for his plight or understanding of his actions. Instead, Grendel is evil because he must be evil; no other option is possible.