Sexuality
Or rather the lack of it.
I think there is something interesting here.
Conventionally, we would expect a 14 or 15 year old boy who takes a girl home and has her alone in his room to make some sort of sexual advance, or at least for events to lead in that direction.
Instead, he kills her.
He later moves her dress so as to make sure she's decently covered, which seems to display a total lack of interest on his part.
Also, I think his murder of the girl, which allows him to experience 'the real thing', acts as a sort of coming of age for him, whereas traditionally sexual conquest is a major part of the 'coming of age' of a young man.
Discuss?
Some velvet morning when I'm straight...