Grendel is actually called a niþgrim (nydwracu niþgrim, nihtbealwa mæst), which is translated as nithing in at least one major translation (need-wrack and grim nithing, of night-bales the greatest), but isn't niþing itself. I had the same thought as you, that I had heard it in Beowulf, but upon the note below about it not being in Beowulf I checked the original. Sadly, I don't know enough OE/Norse to know how closely the words are related/connotated.

But really, even if you ignore the Norse meaning of a demon in human form, a non-person, I can't say I find the implications of "someone who could and should be killed on sight" to be much of an *improvement*.