The poet explains that Grendel and his mother are the descendants of the Biblical Cain, which suggests not only that they are part of a larger religious or supernatural scheme of evil, but also that they are connected with one of the worst things possible in tribal culture – fratricide, or the killing of a brother. However, at other points in the poem, Grendel seems less like a Biblical figure and more like a ghost, a demon, or something else that belongs in a Halloween-themed horror movie.
Critics also like to play with the idea that Grendel might represent something that isn't supernatural at all – a member of another tribe, an outcast, or a warrior who won't play by the rules. After all, the real problem with Grendel is not that he kills people. Pretty much everyone in this story kills people. The problem with Grendel is that he seems to kill for fun and he won't pay the death-price, the treasure that he should give to the Danes to make reparations for the lives that he has taken. So, it's possible to see Grendel, not as a fantastic monster, but as a monstrous human warrior with a pathological love for violence. Or, to spin it another way, you can read Grendel as a vilification of "the other," a demonic representation of someone outside the tribe. Of course, since he feeds on the corpses of his victims, that makes him a cannibal. But maybe that just adds to the chilling horror of it all.