Back when I studied Beowulf at the University-That-Shall-Not-Be-Named, in the Course-That-I-Failed, we interpreted the passage as means that monsters, "who made war with God a long time", are collectively the descendants of the banished Cain, including Grendel.

I am nowadays personally inclined to think that the descendents of Cain are human, but some of them have/can become monstrous and twisted, and that is what has happened in the case of Grendel. But then, I'm also a fan of Vampire: The Masquerade, wherein vampires are the literal descendents of Cain, the first vampire.

So maybe I'm makin' shit up.