“I’m guessing you wouldn’t want to do that to me, as I’m 14,” recites Stinson Hunter as he types the words into his phone. “And then I’ll put a sad face in there for him.” He’s texting a 29-year-old would-be paedophile he doesn’t know, in the hope of arranging an encounter with him.

Stinson Hunter isn’t 14; he’s 32. He’s also changed his name. He used to be called Keiren Parsons, and spent a troubled youth in care. These days, Hunter dedicates himself full-time to engaging, ensnaring and humiliating predatory paedophiles online.

The Paedophile Hunter (Channel 4) documented Hunter’s vigilante project in unsparing detail, resulting in the kind of film that restores your faith in absolutely nothing.

Operating out of Nuneaton, Hunter and a few cohorts set up online profiles for underage girls, and then wait to be contacted. In conversation with his targets, he always makes it clear his fake-self is underage, but otherwise does what he can to keep men interested and talking. In a sense, he grooms the groomers. He gets sent a lot of pictures of penises along the way. If possible, a meet is arranged, then Hunter ambushes his quarry, filming the result and posting it online. A package of evidence and a statement is sent to the police.

There is no element of this that is not disturbing. Although the evidence is largely anecdotal, it would appear that online grooming is commonplace, and that snaring paedophiles through subterfuge is like shooting fish in a barrel. The business of vigilante justice makes for an unedifying (albeit gripping) spectacle; the techniques involved, if they skirt the legal definition of entrapment, are certainly morally suspect. And the online humiliation of would-be predators is, frankly, unpardonable.

Stinson Hunter was a spectral central character. He didn’t just appear in this documentary, he haunted it. His face, wreathed in cigarette smoke, seemed to change shape with his mood. At the start, I thought two different people were being interviewed – only his lip ring made him identifiable from one shot to the next. He looked a wreck, but that turned out to be because he’d recently been run over by a fleeing target.

Talking to camera, Hunter was thoughtful and menacing by turns, with a tendency to be offhandedly alarming. “I could beat her within an inch of her life and she would come back to me,” he said of his dog. “Not that I’d do that.”

The motivations behind his mission remained murky. Asked by the interviewer if he’d ever been abused as a child, he said: “No comment. I’m not talkin’ about that shit.” There is definitely a sense that he harbours a special hatred for predators (though his tactics amount to a kind of predation; it’s just that his targets don’t, in his eyes, deserve any mercy). But he seemed more bent on making up for a past that included a lot of non-specific bad behaviour. “I’ve done a lot of shitty things,” he said, admitting he spent his youth pissing his life up the wall. “Then this happened. I’ve got a purpose.”

There is undoubtedly an adrenalin rush that comes with ambushing would-be paedophiles – you felt it, horribly, as a viewer – but Hunter maintained a surprisingly professional demeanour with his targets. He promises no violence, identifies himself as an undercover journalist and explains everything clearly. However, when you simultaneously film and question your subject while pursuing him through the streets of Nuneaton, where Hunter and his work are well known, it’s not long before something like a mob starts to form. It was easy to imagine the worst happening.

In fact, it wasn’t necessary to imagine the worst. In 2013, 45-year-old Michael Parkes killed himself shortly after being exposed by an unrepentant Stinson. “He made the choice, not me,” he said. “Should I feel bad? That’s the thing, should I? I don’t know.”

While it’s claimed that Stinson has a better conviction rate for predatory offences than many police departments, it was clear that the official imprimatur of an arrest and conviction meant a lot to him. When he heard that one of his targets had received a one-year suspended sentence, he burst into tears. “You don’t expect it with my history of being a complete piece of shit,” he said. After the trial, the police sent him a letter telling him to stop what he was doing.

It’s impossible to dismiss Hunter’s commitment to his mission, or even his methods – the police, after all, employ similar tactics, when they can marshall the resources – but Stinson’s attempts to shed light on this issue have taken him to a very dark place, and for an uncomfortable hour, we were obliged to share it with him.