you can't help it.
I found it is better to watch this movie as a spectator rather than as a participant, because if you allow yourself to become involved in the story, the inconsistencies will annoy you to no end.
The whole theme about cruelty to animals holds no weight because we never become involved with any animals - in fact the stepfather seems to be more of a true animal in the sense of doing what is necessary to survive than any other character.
Christian Bale wavers from being profoundly retarded one minute to quite normal the next. John Hurt shows no remorse for his past deed, indeed is rather demented in confiding that he did the right thing in his little fairy tale retelling.
The main thing that will keep you watching is to see what will happen next. Nothing happens as it should - oh, the basic good will triumph over evil theme is there in all its faded glory, but what will keep you watching this movie is because it is exactly like roadkill. You know it's bad, but you just have to see what it is...