In the book, Oscar thinks about the other side to his father, which he calls The Werewolf--the cloying, selfish, judgmental, self-pitying creature who gives Oscar the creeps.
There's actually quite a bit of fanfiction about both films out there. Some are fascinating (one is a crossover where the killer from The Lovely Bones meets Abby--to his ultimately fatal shock). A few are deeply depressing (with Oscar devolving into a serial killer) or touching (such as Abby and Owen finding a way to feed without killing, or to kill only the evil a la Dexter).
Just showed Let Me In to my best friend. She said she didn't know who's side she should be on, but that she sympathized with every character without believing she'd make any of the same choices.