The reveal made no sense...SPOILERS
As the cop said early on, the crimes took planning and organization, which are pretty clearly beyond the capabilities of the mentally retarded Claeren. Same for scaling walls to peer in windows, then vanishing in a flash when the kids scream. Can you really see Claeren, obese and about as coordinated as a water buffalo, managing that? He was not believable as the killer.
Have to say, though, that he was no more unbelievable than the cop who would endure years of harrassment from his brother's kidnapper without taking action. Or who'd negotiate with the woman he'd seen on video abusing kids, instead of just leveraging - or beating - the info out of her. Most passive cop I've ever seen in a movie.
All together, a weak movie. Only part that had any punch was that ending.