I guess "monster" is too strong word.

But I felt very uncomfortable reading about Minty's dark thoughts and desires, he even scared me. Probably it was caused by fact that I saw movie first with William as more gentle Minty and the difference between two Mintys was too big, IMHO.
I enjoyed the book very much. I have the feeling though that if I read the novel before I saw the movie, I would like it even more. I was obviously "spoilered", I knew how the story ends, all mysteries, etc. It was rather like extended version of the film, with some new scenes and lot of extras about the characters. When you discover all the story for the first time by reading and imagining, it's much more entertaining and it's an unforgettable experience. But if you already know all the story, it's not that strong.
I love good stories about common people and their lives, "Empire Falls" is perfect for me.