This is a tricky question. It is traditionally accepted that Grendel is evil while Hrothgar and Beowulf are good. These aren't moral absolutes; Hrothgar kills many to maintain power, and Grendel attempts to change his nature, but they essentially fall on these sides. Also, Grendel recognizes himself as a monster. He repeatedly speaks of his bloodlust, his meaningless attacks and need to kill, etc. He also knows that he is apart from men, and can never be connected to them.
Despite all this, Grendel is also a nihilist. As he says "Nihil ex nihilo" or "Nothing from nothing." Because of this philosophy, he sees no real morality in the world. There is no purpose, no gods, just existence. So while he identifies as a monster, and often berates himself for his violence and slaughter, he doesn't really see himself as evil. Nor does he consider the men as necessarily evil either. Stupid, yes; delusional, certainly; but not forces of good or evil.