Way way way way back in college we read Beowulf in the original Old English.
Critics proclaimed the 1971 novel from the monster's point of view, "Grendel "by the late U. of Buffalo professor John Gardner as first rate.
Not too long ago I saw a version of the epic on the Science Fiction channel; it was extremely well-done, and had excellent production values esp. considering that this was a cable channel. I haven't seen the current Hollywood version, but I've seen still photos and read reviews, so to answer the q. as to whether the movie deviates from the poem: I don't think it
accurately follows the original. One thing I do know is that Grendel's mother didn't look ANYTHING like Angelina Jolie.