Monsters are pretty damn scary. Horrible, bug-eyed, slobbering, and in no way ever human. Therefore, it's all the more unsettling when it's revealed that a monstrous creature (almost never the main character) was once human, but became a monster through some sort of infection, curse, sheer personal evil or transformation. And there's no means of changing them back. Sometimes the mind is not affected; only the body is. And sometimes the original mind can be reached. This does not undermine the horror of it. Indeed, in certain ways, it makes the horror even worse.