I Am Legend scrapped its original ending because test audiences 'felt cheated' by the big twist
Discussion
(winteriscoming.net)
Yes, the movie has a version of infected fast zombies. The book had vampires.
Also, I didn't think the main character of the book was monstrous. The other book characters should not of thought that either. The vamp faction that caught him was actively hunting other vampires that were not part of their faction, so no one should have been shocked.