SPOILERS SPOILERS SPOILERS
I didn't get any feeling of Oscar from Let the Right One In being creepy, exept in the way that *all* children are creepy. In fact, the boy who played him seemed very sweet and innocent, at first. He had a caring (if oblivious) mom, and a dad who he could rely on if he was sober (which is better than nothing). He collected newspaper articles about serial killers and played out revenge fantasies in the court yard with a knife, but these, to me, seemed like normal adolescent habits and behavior. If not for Eli, eventually he would grow out of it (once puberty kicks in).
While Kodi Smit-McPhee's Owen in Let Me In seemed like he was already at a cross-roads, even before Abby gives him the extra push over the edge. The bullies are much worse, his parents' divorce is way messier, his *mother* is an alcoholic and the only thing his father is good at is picking fights with his mother. Owen states at one point that someday he'll leave and 'never come back.' The movie paints a portrait of a kid who is trapped in a steadily worsening situation who will not survive his childhood intact. He'll either run away from home, or kill himself. Which all builds into the idea that Abby is his guardian angel.