America's Serial Killer: True Stories was a little baffling. There's that title for a start. America only has one serial killer right now? That seems implausible, and in any case you'd think people might be wary of stirring the murderously competitive element that sometimes seems to drive serial murderers. Or was it supposed to suggest that this serial killer – an unknown man who has been murdering sex-workers and dumping their bodies on Long Island – is archetypally American in his modus operandi? And then there's the film itself. What are we supposed to do with it other than feed our appetite for narratives of depravity? Perhaps that, and that alone, is the point. Audiences find serial murder fascinating, and although this particular story can't supply the catharsis of a monster brought low (the perpetrator is still out there somewhere), it can scratch our itch to know the worst.

Not only can the police not find the killer, but the tabloids can't even decide what to call him. Some settled for the Gilgo Killer, after a beach near where the bodies were found, while others went with the Long Island Serial Killer or the Craigslist Killer, because of the fact that many of the victims are believed to have arranged to meet him through the classified ads website. His existence first came to the police's attention after the disappearance of a young woman called Shannan Gilbert, seen running away in distress after a meeting with a client in a local gated community. Searching for her, the authorities found four bodies dumped just off an oceanside highway. None of them were Shannen, but eventually the body count rose to 10, some of it horribly tallied in isolated body parts.

Louise Osmond's film had one significant difference from a standard cable-channel penny dreadful. It concentrated for its interviews not on investigators or researchers but on the victims' relatives and friends. And its subject was less the process of the police enquiry (which has got nowhere, after all) than on how these women were missed – the unanswered phone calls and growing silence that led those who cared about them to call the police. The police didn't always listen very sympathetically. At the time, the NYPD reportedly didn't even take missing persons reports for prostitutes until they'd been gone for 10 days, which is a hell of a head start for a murderer. And the really unsettling thing about the film – apart from the sadness of the relatives and apparent absence of fruitful leads – was the pervasiveness of male violence that lay short of murder.

So many of these women were coaxed into the business by men who flattered and then exploited them, and beat them. One of them even appeared on camera, slouching in baseball cap and shades, as he explained how he'd come to hit his girlfriend so hard that she had to have a steel plate inserted in her jaw. That was Shannen, who sadly did later turn up, as bare bones in the scrubland. Meanwhile, Amber Costello's sister, who actually introduced her to escort work in the first place, still places ads on Craigslist "hoping I'll catch his eye". I hope she doesn't, but I don't doubt there's plenty of others out there who'll be happy to hurt her in lesser ways.

Protecting Our Children isn't exactly a pick-me-up for the battered spirit either, but it has been an excellent series, detailing the (literally) thankless task of preventing inadequate and damaged people from passing their damage on. Last night's clients included a drug-dependent mother, Louise, who eventually surrendered care of her second daughter, just as she had with the first. There were tiny consolations – babies rescued and responding to proper care. But there was also a reminder, right at the end, that these genuinely dedicated social servants are bailing with a sieve. "Louise and Wayne are expecting a baby boy," read a credit caption.