Wrote the following post in 2012 after seeing the film version of John Ajvide Lindqvist's book Let the Right One In. After reading the book I'm not so sure of this. More after the blockquote. [Spoilers]
Let the Right One In [the film] is a perfect loop that spins out more even metafiction than the main story contains.
Several mysteries of the clumsy Father, surrogate Father, or captor seen in the first half are explained in the second.
The Father, we learn, is the boy at the end of the next cycle of serving as keeper/guardian for the ageless vampire girl.
What strikes us initially as the Father's slow-witted ineptitude is in fact burn-out and grief after a lifetime of murdering for her and covering up her crimes. He still loves her, because she once seduced him just as convincingly and decisively as she does the Boy in this film. Yet he longs for death, wants to get caught, and disfigures himself horribly when he sees he is about to be replaced, inevitably, by a younger guardian.
All of this will happen to the Boy, as it happened to unknown other boys before. We are seeing the beginning and the end of his life.
One critic complained about the violence of the revenge in the swimming pool at the end -- was it just a cheap thrill for the audience? Perhaps, but the pleasure is hollowed-out by the scenes of the Boy weeping afterwards. Also the extremity of the event further explains the Boy's willingness to give the girl decades of servitude -- he owes her big time. Prior to this we saw him vacillating over her murders, even losing his taste for his serial killer clipping collection. After this incident, he's hooked for life.
I pondered the gender-bending of the vampire Girl. It explains how/why she offers "guy advice" to the Boy about defending himself from bullies. She asks the Boy to "be me" but also wants to be him.
We see hints of how the power dynamic of this very alike couple will play out over years of the Boy's servitude. The girl bosses the Father around and occasionally offers him a stroke on the cheek. The Boy, feeling his oats after shellacking his first bully, plays games with the girl's weakness of not being able to enter a room uninvited. She must give him a bloody demonstration of where such games will lead.
Most the reviews I skimmed talked about the coming of age/romance aspects of the story but not its exposition of the roots of a lifetime co-dependent relationship.
Reading Lindqvist's novel, source of the film, several years later, lowers the above interpretation a few notches. [More spoilers] In the book, the "father" is an alcoholic with a jones for boys, picked up by the vampire late in the alcoholic's life, and the vampire is in fact a boy, missing genitalia since his transformation to bloodsucker instigated by a sadistic vampire aristocrat in centuries past. (The purpose and mechanics of the de-sexing are a bit murky in the book.) Although Lindqvist wrote the script for the film version, the decision was made to downplay the sexual elements. Those changes certainly still leave open the interpretation above -- that in the film, the Father was once a Boy to the vampire, and the story hinges on the acquisition of a new Boy. Nevertheless, this spin was not in the author's mind. I still like my "loop of doom" version but it may have to be reclassified as fan fiction.]
Update: A.V. Club critic Scott Tobias had a take similar to mine:
And then the coda -- which finds Oskar on a train during the daytime, tapping Morse code to Eli, who’s curled up in a box by his side -- feels remarkably bittersweet. Their destinies are now entwined, and they aren’t alone any more, but for how long? Someday, Oskar will also be a middle-aged man, trudging out in the snow with a funnel and a jug, collecting sustenance for his beloved.