Hakan, Eli's companion, in Let The Right One In has a rather interesting purpose to serve. We all know that whether it be from from the story of Dracula or popular media that vampires need blood but why? The audience only views his role in Eli's life but unsuccessfully. His job is to obtain blood from innocent people so that Eli can survive but why? Is he doing the right thing?
He kills by hanging the body upside down so that the blood inside the body will drain through the neck without damaging any of the bodies internal organs. Also the gravity aids the speed of the draining process so that it only takes a matter of seconds. He does this a few times in the movie, which goes to show that he has done this before and he shows no hesitation. He is even willing to let Eli suck his blood when he is on screen for his final scene. There is the possibility that he would prefer to do the killing so that Eli, a "child" doesn't have to. It isn't possible that he is Eli's father because she has "been twelve for a long time" and Hakan is just an ordinary man.
Haken also gives no indication on why he is killing for the sake of another, which is intriguing but the movie does not provide the answer. Eli could be his daughter or a friend but the motivation is never revealed. However, he is not killing people for no reason; there is a psychological reason for this. To kill so many others, Hakan must have a personal stake in whether or not Eli survives or not. His motivation proves to be deep or else he wouldn't have poured acid on his face to avoid going to jail.
Was Hakan doing the right thing? Perhaps, but all the audience can do about this is speculate.