Let The Right One In

Let the Right One In was one of the most haunting films I have ever watched.
The narrative itself I found very uncomfortable to watch - particularly the relationship between the lead character and his school friends. As the boy was portrayed as potentially quite emotionally disturbed, this made it more harrowing to watch as his school friends taunted him. It set up the idea that perhaps the boy would react violently due to his unstable state of mind.
His relationship with the girl, however, is also uncomfortable although I was not entirely sure why. The girl seemed to act much older than she is which made her intentions with the boy confusing. It was unclear to me whether she really just wanted his friendship, or something more.
When it was revealed that the girl is in fact a vampire, it gave the all the characters a real sense of depth. It was now apparent the the girl must be deeply lonely in her vampire life, and that she genuinely does want a friend. But this is mirrored in the fact that she is a killer - does this affect the audiences empathy for her? Do we feel more sorry for her because she is a little girl?