‘I didn’t think all of these people would care,” Steven Avery says, in wonder, at the beginning of the trailer for Making a Murderer: Part 2.

But people did care; they cared a lot. When the first series of Making a Murderer launched on Netflix in 2015, millions of people around the world were transfixed by the true story of Avery, a Wisconsin man convicted of murdering a local photographer, Teresa Halbach. There was a frenzy of interest about whether Avery had killed Halbach or whether he had been, as the series seems to suggest, a victim of police misconduct. Making a Murderer quickly became a bona fide cultural phenomenon, arguably the biggest true-crime documentary of all time.

Not everyone was thrilled by the documentary’s success, however. “I’ve had 4,000 death threats since Making a Murderer first aired,” says Ken Kratz, the prosecutor who helped put Avery behind bars. “I’ve had packages explode in my office. I’ve had my car shot at.” He sighs. “I suspect all that craziness is going to be unleashed again.” The sequel to Making a Murderer comes out on 19 October and Kratz is apprehensive about what news it could contain. “Their tag line is something to the effect of: ‘The case is not over yet,’” he says. “Well, when is it over? From my perspective, this case is over.”

Kratz may have had enough of Making a Murderer, but the rest of us clearly have not: the sequel has already drawn extensive press coverage. And it’s not just Making a Murderer. It seems as if many of us can’t get enough of murder, full stop. In recent years, true crime has become a pervasive part of popular culture.

A lot of the credit, or blame, lies with the podcast Serial, which followed the case of Adnan Syed, convicted of killing his ex-girlfriend Hae Min Lee in 2000. When it launched in 2014, Serial smashed all podcast records. The first season has been downloaded more than 211m times and a third, which focuses on the Cleveland court system, launched last month. It has also been turned into an HBO show, The Case Against Adnan Syed, coming out soon.

True-crime successes continue to come thick and fast. Last year, for example, the LA Times podcast Dirty John, which details the violent web of deceit spun by supposed “freelance anesthesiologist” John Meehan, was downloaded more than 10m times in six weeks. It has also been turned into a TV show, starring Eric Bana, which will premiere next month.

Meanwhile, there are true-crime TV channels such as Investigation Discovery, with a nonstop schedule of shows such as Evil Twins, Evil Stepmothers and Evil Lives Here. For those who prefer a more hands-on homicide experience, there’s an annual convention, CrimeCon, where you can mingle with other murder aficionados at events such as Wine & Crime or test your mettle at an interrogation experience. You can also shop for serial killer swag on Etsy, which boasts a disturbing amount of murder merch, from coffee mugs decorated with names of famous killers to blood-splattered hair-ties.

Michael Arntfield, a former police officer who now runs a cold-case thinktank, notes that our interest in “ripped from the headlines” stories of depravity is not a modern phenomenon. The genre, Arntfield says, really “crystallised in 1842 when Edgar Allan Poe wrote The Mystery of Marie Rogêt”, a short story based on a contemporaneous killing. Since then, Arntfield says, the genre “has been intermittently influential and has come and gone generationally”. There has been nothing quite like the quality and quantity of attention we’re seeing today, however.

Once a guilty pleasure associated with rubberneckers and cheap, gory magazines, true crime has “moved out of the gutter”, says Jean Murley, author of The Rise of True Crime: 20th-Century Murder and American Popular Culture. “It hasn’t necessarily become highbrow entertainment, but it has a lot more cultural cachet. People aren’t ashamed of liking it the way they were 10 years ago.” In a virtuous circle, a rise in high-quality true-crime content has created a wide audience, which means that more high-quality content gets made.

Technology has also assisted the gentrification of gore. As Arntfield notes, new production platforms such as Netflix allow for “greater experimentation with long-form storytelling. Rather than telling a whodunnit in an hour, shows like The Staircase and Making a Murderer have taken true crime in a new direction which is more experiential.”

The Staircase covers the trial of the novelist Michael Peterson for the murder of his wife, Kathleen, who was found dead at the bottom of the staircase in their North Carolina home in 2001. Eight episodes on the original trial were released on French TV in 2004. Another two episodes followed in 2013. After a retrial, three new episodes, together with the 10 previous, were released on Netflix this year. A story spanning almost two decades was condensed into an extremely bingeable but also nuanced series.

As well as changing how true-crime stories are told, technology has democratised who gets to tell them. As Arntfield says, it is relatively easy for anyone with a knack for narrative and an internet connection to dig up an interesting cold case and turn it into a podcast. “Access was always the issue before. Unless you worked on the original case, you didn’t have access to the information you needed to tell these stories.” Now, however, “we’re realising how many stories are out there. There’s a limitless amount of material.”

Should it all be used, though? These aren’t just stories – they are real people’s lives. No matter how tastefully it is done, is it not unethical to transform personal tragedies into public entertainment?

It depends on how you define entertainment, says Christopher Goffard, the host and creator of Dirty John. “My aim is storytelling that explores important psychological questions with a respect for human complexity and ambiguity … To insist on hard lines between journalism and entertainment is to assume that journalism has to be boring or it’s not authentic, which I don’t buy.”

Goffard also notes that true-crime stories can sometimes be more powerful than traditional journalism. “One issue at the heart of Dirty John is something called coercive control, which is a form of psychological manipulation that involves things like gaslighting, microsurveillance and isolation of a domestic partner – control that masquerades as love. I could have done a story quoting a handful of people who have endured this, and found some experts to talk about it, and it would have been a respectable story and maybe sparked some conversation. But I think the effect is hugely magnified when the story takes you deep inside one family’s experience, so you get to hear what it felt like to live it.”

Others, however, seem to have spent less time pondering the ethics of true crime. Take Payne Lindsey, the host of another hit podcast, Up and Vanished. In the first episode, Lindsey explains the genesis of the series, which examines missing-person cases. “Like a lot of people, I had been pretty obsessed with the podcast Serial, and the Netflix series Making a Murderer, and I thought to myself: ‘What if I made one of those?’” he says. “So I literally just went to Google and started searching.”

The satirists of the Onion parodied this sort of self-absorbed approach in a podcast called A Very Fatal Murder. Released this year, it features “David Pascall”, a narcissistic Brooklynite who decides to parachute into Bluff Springs, small-town America, to solve the death of a pretty young girl called Hayley Price and maybe win some awards in the process. “So, what happened to Hayley Price?” Pascall asks. “And how can I get in on it?” Katy Yeiser, the head writer on A Very Fatal Murder, notes that among the many true-crime tropes ripe for mockery is the “self-aggrandising host exploiting a young woman’s death”.

Sometimes, of course, true-crime programmes succeed where the authorities have failed and give a voice to victims who have been silenced. These stories aren’t just being told, they are being re-investigated. The internet allows people to trade theories, hunt down clues and influence the narrative.

One of the best examples was last year’s Netflix documentary The Keepers, which examined the unsolved murder in 1969 of a Baltimore nun, Sister Cathy Cesnik. The series follows two of her former students, now in their 60s, as they investigate how she was failed by patriarchal systems of power, from the Roman Catholic church to the Baltimore police force.

The “Grandma Nancy Drews” Gemma Hoskins and Abbie Schaub had been working with the journalist Tom Nugent to try to draw attention to the murder of Cesnik years before the documentary was made; only in 2014 did some film-makers came knocking. The current obsession with true crime, you could argue, helped propel Hoskins and Schaub’s years of overlooked industry into the national spotlight.

Hoskins has worked hard to maintain the interest that The Keepers has sparked in the Cesnik case, using social media to crowdsource clues for the investigation. She’s in the middle of following a new lead when I call her at her Maryland home. “There were supposedly two hunters hunting small game who found Sister Cathy’s body. They’re only named in one article. I want to find them before they’re both dead.” Hoskins has a team of about 30 volunteers, recruited via Facebook, trawling the internet in an attempt to identify these hunters.

Already The Keepers has led to important developments in the case. Shortly after the documentary was released, the Baltimore police department started to offer an online form for victims of sex offences related to the events it covered. “It’s surreal,” says Hoskins. “I’m thinking Pope Francis probably knows who I am! It’s hard for me to grapple with the fact that I have a voice now and that people are listening.”

Australian podcast The Teacher’s Pet is another example of true crime making a difference. The podcast, which launched this year and has been downloaded more than 24m times, looks at the disappearance of 33-year-old Lyn Dawson from her home in Sydney in 1982. Nobody was charged in connection with the disappearance.

Hedley Thomas, the creator of The Teacher’s Pet, says the information that came in after episode one of the podcast aired changed the course of the whole series. “Very quickly people started contacting me, wanting to share information and things they’d witnessed, so I had to rewrite the next two episodes.” The fact that The Teacher’s Pet wasn’t entirely scripted but unfolded week by week, he says, “made a big difference to the material that I started to discover”. It made listeners feel that they were an active part of the investigation.

“When I first went to the police to see if they’d cooperate,” he recalls, “they didn’t want to have anything to do with the podcast. But during the series, people were contacting me that the police hadn’t heard about. Suddenly there was this change of attitude. The police commissioner himself knew he had to utilise the momentum of what was happening and get new information into the hands of investigators.”

Lyn Dawson’s family, says Thomas, are delighted by the success of the podcast. “Lyn’s brother has said that the podcast has given them the best hope they have ever had that this case will lead to a prosecution, which should have happened years ago.” However, not all victims’ families are so thrilled that old wounds are being reopened. While stories such as Making a Murderer, Serial or The Staircase, which seek to exonerate convicted killers, have been praised for exposing flaws in the criminal justice system, they have also been accused of exploiting the deaths of the women involved and preventing their families from getting closure.

In 2016, the public interest that Serial created in the Adnen Syed case resulted in his conviction being overturned and a judge ordering a new trial. While many Serial fans were ecstatic about this development, Hae Min Lee’s family lamented the fact that it had “reopened wounds few can imagine”. In a rare statement the family said: “It remains hard to see so many run to defend someone who committed a horrible crime … when so few are willing to speak up for Hae.”

Appeals by prosecutors have delayed Syed’s retrial, but it is looking likely that it will happen soon. This will, no doubt, cause a new surge of media interest and make it even harder for Lee’s family to make peace with the past. The same is true of the sequel to Making a Murderer. While Halbach’s relatives have largely remained quiet about the Netflix series, they have said the show traumatised the family all over again.

Whatever the ethical arguments about true crime, its popularity seems unlikely to run out any time soon. “At some point down the line, using a podcast or serial documentary to tell true-crime narratives will become less trendy,” A Very Fatal Murder’s Katy Yeiser says. “But true crime will not go away. It will just be told through some other or new medium. We will never grow tired of murder.”