From Ain't it Cool:
"According to Avary, he and Neil Gaiman went to the original epic poem and did as literal a translation as possible, with the only liberties filling in holes in the story, either gaps in time or something left unexplained...In this instance, they realized that Grendel was the son of the demon and The King, which would be the reason he's tormenting his father, as well as dragging living men off to his mother."
Uh, sure. And the gold bikini on Angelina Jolie (AKA Grendel's mom) -- what's the Old English for that? Se gylden brydbosm?