Let me in

I've just watched a movie called 'let me in'; it's kind of horror or romantic. And I'm still thinking about this movie. Trying to find out what kind of feeling I got from this movie. Main plot is that a young-looked vampire girl is falling in love or getting his new helper boy who is 12 years old. She also looks the same age as the boy, telling themselves that they like each other. Most interesting thing is that the old man ,I thought he was her father, becomes the little boy who is on the picture with her. It means that the old man lived with her for his whole life, feeding her with fresh human blood he killed. Most pathetic thing is that he loves her, as I believe, and burn his face with acid then let the girl to suck his blood. Does it mean that the new little boy also has got the same fate as the old man.
Now I can recall the feeling from the 'Shutter Island' is the same. I really want to meet with their author and ask how they got this idea.