‘There’s no guide book for how to deal with this,” says Lenny Royal on his life since being maliciously infected with HIV. Last year his attacker, Daryll Rowe, became the first person to be jailed for using his illness as a weapon by deliberately infecting Royal and four other men with the virus, and exposing five others to it.

Royal, 38, is an engaging and personable makeup artist whose celebrity clients have included Sigourney Weaver and Hailey Baldwin. The day after our interview, he will create glamorous looks for the Brit Awards. He is used to operating behind the scenes, but has waived his anonymity to appear in a new BBC documentary about the Rowe case. He explains that his purpose in contributing to The Man Who Used HIV as a Weapon is straightforward: “There was all this video of Daryll, Daryll, Daryll, Daryll and there was no voice for any of the victims.”

Despite its Rowe-centric title, the documentary tells the intimate, often poignant stories of Royal and four other gay men from around the UK who had the misfortune to meet Rowe online. It is the work of first-time director Charlotte Charlton, who says at the outset that her film constitutes an attempt to provide these men with some answers. She interviews Rowe over the phone in prison and spends time with his foster parents. The bewildered Jacqui and Harry first took Rowe – now 28 – into their home in North Berwick, East Lothian, when he was a small boy with scars on his body and a collection of 120 Barbie dolls. Jacqui says that as a child Rowe was “good company ... a really lovable boy”.

Royal, who used to live in Brighton but is originally from New York, encountered Rowe, a handsome hairdresser, on the Grindr app in 2015. His 12-year relationship had broken down and Rowe was the first person he met on the app. “I was like, let me have sex with somebody else and maybe I can get over this depression,” Royal tells me.

There was an exchange of flirtatious messages and images before Rowe called Royal from a withheld number and announced, “I’m outside your door.”

This was peculiar. Royal had blocked Rowe on the app because of Rowe’s suggestion that the pair should have unprotected sex. “But I had already given him my address, and when he showed up, I let him in – something I obviously regret now,” he says.

Daryll Rowe before sentencing at Brighton crown court. Photograph: Gareth Fuller/PA Wire

After a couple of hours of banter, the pair had sex. Rowe wore a condom at Royal’s insistence, safe sex having been non-negotiable on account of his devastating family history. “My parents died of Aids in the 80s,” he explains in the documentary over faded photographs of an attractive young couple. “My dad was a junkie, contracted HIV through needles and passed it on to my mum. It was a death sentence then. It was a very scary time; I knew that they were going to die. I grew up saying I would never, ever, in a million years, let this happen to me. I never wanted to utter those words that I had HIV – I never put myself in a position where I would be exposed to that.”

After the sex, says Royal, “we watched stupid YouTube videos of a cat or something” and Rowe talked about his veganism and his passion for juicing, mentioning that he was staying with a family in Saltdean, a village on the eastern edge of Brighton. Royal had dinner plans. “I was like, OK, you need to go now,” he recalls.

Subsequently, as Royal puts it, “things got a little weird”. Rowe sent messages that were by turns possessive and xenophobic. Royal blocked him again, this time across multiple social media channels.

The following Wednesday, Royal received another anonymous call. “How dare you block me, stupid American,” said the voice, which then revealed itself to be Rowe. “You can’t get rid of me … you’re gonna burn … I ripped the condom.” Royal told himself this was just the unpleasant invective of someone who felt rejected.

Two weeks later, Royal started to feel ill. He began a course of antibiotics, before a sexual health check revealed he was HIV positive. “I felt like that was it, my life was over,” he says. “I was crushed.” After his parents died, he had been adopted and raised by his older cousin and his wife. They, too, were devastated by the news. “My [adoptive] dad felt like he’d failed in protecting me and that’s the thing that he had promised my mom to do before she passed away. [I told him] it’s not your fault – it’s not like I’m going to have my dad protecting me while I’m having sex, but he cried.”

Attending the clinic the day after his diagnosis, Royal told the nurse what had happened with Rowe. “She was like, I shouldn’t be saying this, but you’re the fourth person who has said the same exact thing,” recalls Royal. His determination that the police should be contacted, and the information he was able to supply them, resulted in an address in Saltdean being raided. Inside, Rowe was in bed with another man – his eighth known victim – but an examination of his phone revealed that hundreds more unsuspecting men had been targeted using dating apps. All of them had to be contacted and told of possible HIV infection.

Footage of Rowe’s calmly deceitful answers during his interrogation by Brighton police features in the documentary. He was bailed to his foster parents’ house in Scotland on condition that he immediately started HIV treatment to reduce his viral load to untransmissible levels. It is now known that Rowe had previously declined safe and effective antiretroviral medication in favour of drinking his own urine, which he considered to be a cure. YouTube footage exists of Rowe vlogging enthusiastically about the supposed benefits of coconut oil.

What happened next is worthy of Rebus. Rowe absconded from his parents’ home and went to ground in the Pentland Hills above Edinburgh, where police discovered an empty tent containing his prescription medication. A nationwide manhunt was launched in August 2016. By this point 22 men around the country – several of whom had been taunted with messages from Rowe, disclosing his HIV status with a barrage of laughing emojis – had spoken to the police about him. He had fled to Newcastle where, using the pseudonym Gary Cole on the dating site Plenty of Fish, he inveigled his way into the home of Tom, another contributor to the documentary, who discovered lacerated condoms among Rowe’s belongings. Rowe was eventually arrested there.

It was in March 2016, while Rowe was still on the run, that Royal went back to the US to be close to his adoptive parents and he hit rock bottom. “It was the classic Hollywood movie situation,” he recalls. “I checked into a hotel and I was going to end it all, but then a friend happened to call just at the right time.”

He returned to the UK to give evidence in Rowe’s trial, which he describes as gruelling, mainly because of what he felt was the defence barrister’s homophobic line of questioning. “The questions she was asking were basically just making sure I was shamed in front of the jury,” Royal says. Unlike other witnesses, whom he saw cry in the witness box, Royal was unyielding and unapologetic. “I was like, yeah, I sent him a dick pic, what else do you wanna know?”

At one point during cross-examination, the barrister seemed to suggest that the knowledge that her client was a vegan should have alerted Royal to his HIV positive status. “I looked at the judge and I was like, that’s one of the most ridiculous questions I’ve ever heard. I thought, you are not going to do this to me and I put up a fight with her.”

These days, Royal is on medication and healthy – and intends to stay that way. Dating someone with HIV is less stigmatised than it used to be, he says, partly because of the growing number of gay men taking PrEp, medication that can protect users from HIV transmission. “It’s no longer a death sentence, not even remotely. But it is messed up when somebody makes that choice for you, when they take that right away from you. That’s what’s difficult to get over. It really is a harsh reality when you think about it, and that’s the thing that I struggle with, on a day-to-day basis. How can somebody be so cruel?”

Rowe’s motivations remain largely unclear. Part of the documentary has filmmaker Charlton playing Rowe’s victims a recording of her conversation with Rowe, during which he says he hopes his victims can forgive him, just before incongruously chatting to his foster mother about the DVDs he has been enjoying in jail.

Royal is understandably scathing about the exchange, which makes for an unsatisfying, but astonishing, denouement: “It doesn’t make any sense at all. He obviously has a screw loose because he’s like, oh, I hope they forgive me. No! And then he’s talking about Cougar Town. That really does show where he is, mentally.”

I ask Royal whether he’s going to watch the documentary. “My friends are like, shall we have a viewing party? But I’m not going to get everybody over for martinis and popcorn.”

He then pauses and laughs, puncturing the tension for my benefit. “Actually, you know what? Maybe that’s not such a bad idea.”

I tell him I don’t think anyone would dare to disagree. Like the man says – there is no guide book.

• The Man Who Used HIV as a Weapon will be on BBC iPlayer from 15 March and on BBC One on Tuesday 19 March at 11.05pm.