Part of the thrill of Baby Reindeer was the feeling of watching the birth of a monster. Comedians starring in their first scripted drama tend to base their characters gently on themselves, prodding at their own foibles without doing proper damage – but Richard Gadd set fire to that safety net by dramatising his own experience of being stalked, along with other, even darker moments of victimhood, with an honesty that was transgressive.

On screen and in his old real life, the helpless Gadd’s unhinged admirer Martha (Jessica Gunning) pursued him unstoppably, like the fiend in a horror movie; once Baby Reindeer’s word-of-mouth popularity exploded and Gadd won major awards for playing himself at his most vulnerable, though, his success made him one of the most powerful creators in television. That queasy disconnect was fascinating. The prospect of watching a new Richard Gadd show is exciting, of course. It’s also a bit frightening.

What’s immediately interesting, psychologically speaking, about the six-part BBC iPlayer drama Half Man (from Friday) is that it’s another show about a terrifying black hole of a person ruining the life of someone who shows them weakness. But now, writer-creator Gadd has cast himself not as the target but as the monster. Muscled up beyond recognition and sporting a straggly beard and brutal bowl-cut – a combo bizarre enough to turn its wearer into a horror icon, like Leatherface or Michael Myers – Gadd’s new alter ego is all id. He is vengeance, pure and raw.

This story of two “brothers”, Niall and Ruben, begins on the outskirts of Glasgow in the 1980s. They’re not blood relatives, but when Niall’s widowed mum starts a relationship with Ruben’s divorced mum and invites her to move in, Niall has to share his teenage bedroom with two-years-older Ruben, or at least he does once Ruben is released from the young offenders’ institution he was put in for biting a man’s nose off. For weedy, nervous Niall (Mitchell Robertson), raging psychopath Ruben (Stuart Campbell) is a devil’s bargain. The strong big bro deals comprehensively with the bullies who have ruined Niall’s schooldays and – in the first of many scenes where you can almost feel Gadd daring you to keep watching, when all your instincts are to look away – Ruben directly assists Niall in losing his virginity. In return, Niall helps Ruben cheat in his exams, and generally offers him the kindness no one else ever has.

Exceptional … Stuart Campbell (left) and Mitchell Robertson play the younger versions of Ruben and Niall. Photograph: BBC/Mam Tor Productions/Anne Binckebanck

The two are locked together from that point on in painful symbiosis, an uncomfortably eroticised headlock of a relationship that Niall doesn’t consent to and simultaneously can’t live without. An opening flash-forward has already shown us the adult Niall (Jamie Bell) surprised and shaken by Ruben (Gadd) having shown up at his wedding: Niall is in his jacket and kilt, but Ruben is stripped to the waist and they are alone in a barn, away from the other guests. Not for the last time, Half Man is about to bring you violence so vivid, you’ll think you can taste blood in your mouth.

With the guide rails of portraying real events taken away and female characters mostly relegated to unheeded voices of reason, Gadd’s preoccupation with broken masculinity runs riot. It veers close to pornography. Once again, past trauma doesn’t just explain men’s (self)-destructive behaviour: it makes it inevitable, to the point where their maddening choices are dramatically difficult to accept. Meanwhile, Gadd’s interest in shame as a driver of male misery mixes uneasily with his inability to resist making the sex as shocking as the violence, so that as Niall struggles with his own desires, he rarely has the chance to explore them in a way that isn’t extreme. The dialogue is unsparing, too: across several epic two-handers in which Bell and Gadd give spectacular performances so frank they’re almost feral, both characters are analysed to death. But when Gadd hits a nerve, he still strikes it harder than any other TV auteur.

You wonder where Gadd goes from here. Can he make a third drama on the same themes, even more compellingly horrible than the first two? That would surely be a bad idea. But I probably wouldn’t say that to his face.