I thought that was what made the finale stronger, the fact that the killer's inescapable connection to the tragic childhood event is hinted at with the calls. It makes the ending seem less of a stretch. The argument that you posed about the son acting out because of a failed father figure is interesting. Since the adults could never close this case, and are indeed distracted by other ones related to the children, it is ironic that a child takes it upon himself to follow through with the retribution. Your point would be even stronger, too, if that excised TV cut scene of Nielsen chopping wood with the eventual murder weapon were included in the circulated cuts of the film.