A house should not be a frightening thing. Houses do not get haunted. Houses, unless they are on fire, are incapable of harming you. Yet, standing in front of a small, drab maisonette on the edge of Peterborough – the house in which serial killer Joanna Dennehy committed her first and third murders – I can't help but get the creeps.

People on the Welland estate have no great wish to talk about Joanna Dennehy and the terrible things she did here. Two teenagers on a dirt-bike, tearing up and down on a patch of grass beside the house, wonder why I would care about the woman or her murders.

"She was a psycho, mate," says one.

"A fucking psycho killer, wasn't she?" says his friend. "End of story, mate."

Joanna Dennehy and the Peterborough murders of 2013 still evoke a grim fascination: how did this woman wander through life, doing all the normal things normal people do, and then at the age of 31 stab three men to death for no discernible reason? People cannot, surely, be born this way. So is there anything in Joanna Dennehy's story – or in her strange actions since entering prison – that could help us understand how a human being can go from seemingly calm to so thoroughly cruel?

Dennehy hasn't really gone away, even after being sentenced to life imprisonment in 2014 and locked up in HMP Bronzefield, the category A prison in rural Surrey. The photo of her mime-licking what looks like a Final Fantasy replica knife is lodged in the national psyche, and she's done a remarkable job of keeping herself in the headlines from beyond bars.

In March of 2014, just a month into her sentence, she began exchanging letters with the ex-wife of one of her accomplices, claiming she took the rap for one murder she did not commit. DCI Martin Brunning, an investigating officer who helped bring Dennehy to justice, tells me the letters were reviewed, but that there was simply "no evidence to support [her] claim". Later in 2014 Dennehy reportedly began a pen romance with a builder from West Sussex – a man who told a tabloid paper he had fallen in love with her. Then, in 2016, details emerged of an escape plot: Dennehy had planned to kill a female prison officer and use her fingerprints to fool the prison's biometric door locks. As punishment she was placed in solitary confinement, and later in 2016 tried and failed to win damages in a lawsuit against the Ministry of Justice, claiming her segregation amounted to "inhuman or degrading treatment or punishment".

Plenty of fun and games after only a few years inside HMP Bronzefield, and no doubt there will be more to come in the years ahead.

Psychologists are getting a little closer to understanding the nature of psychopathy and the need to play these games. Various studies have found that psychopaths lack empathy and have a need for self-aggrandisement by gaining power over other people, while others suggest that in their need for stimulation some psychopaths have a capacity for risky behaviour.

"Most psychopaths aren't murderers – they don't have the need to do that," explains Dr Elizabeth Yardley, the director of the Centre for Applied Criminology at Birmingham City University. "And most murderers aren't actually psychopaths. They have some feeling, some conscience, for what they have done. But there is this overlap where you have these very, very extreme cases, like Joanna Dennehy. I think of those kind of psychopaths as mad scientists – other people are their experiment. Like, 'I wonder what happens if I do that.' If you have zero empathy for other people, there is nothing to do stop you acting on the curiosity – even acting violently – under certain circumstances."

So what made Dennehy act? What circumstances led to her becoming a serial murderer?

Joanna Dennehy (Photo SWNS)

Joanna Dennehy grew up in Harpenden, a small town in Hertfordshire. The cul-de-sac where she was raised is certainly quiet and comfortable, if a little dreary. Her sister Maria told the BBC that their parents – her security guard father and shopkeeper mother – worked hard to provide for them, and insisted their childhood was thoroughly normal. "There was a girl we loved (who) turned into a monster," she said.

Yet, serious problems did begin early in Dennehy's teenager years. She was only 13 when she ran away from home, briefly, with a young man her sister believed to have been much older – 18 or 19. She began to steal from her mum and dad. She ran away a few more times, then finally left home for good at 16 with a man five years older than her, John Treanor.

Treanor and Dennehy moved to Luton, then Milton Keynes, then Wisbech in Cambridgeshire. She had two children before the age of 21. She was violent with Treanor, kicking and punching him when drunk. She would leave home for days without explanation. She slept with other people. She self-harmed. And as a prison psychiatrist later discovered, she had paraphilia sadomasochism – a need to give and receive pain while having sex.

Treanor left in 2009 after Dennehy threatened him with a six-inch dagger. He moved north and took the two children with him. Dennehy roamed from town to town in Cambridgeshire and East Anglia. In 2012, a year before the murders, she was given a 12-month-suspended sentence for assault. The same year she also spent a few days in Peterborough city hospital and was diagnosed with having an anti-social personality disorder.

After leaving hospital, Dennehy tuned up at Quicklet, a small letting agent in Peterborough, where she found a bedsit to move into. Quicklet co-owner Kevin Lee soon gave her work doing odd jobs and helping to evict tenants, which gave her access to a number of local houses. Her first murder happened in Welland, at the north end of Peterborough, in one of these houses. Dennehy had met Polish warehouse worker Lukasz Slaboszewski in the town centre, and he'd told friends excitedly about an "English girlfriend". On the 19th of March, 2013, Dennehy texted him and asked to him to come to the Welland address. When he arrived she stabbed him to death just inside the front door, plunging a pocket knife through his heart.

Evidently, the murder did not disturb her. At all. In fact, she showed Slaboszewski's corpse – which she had stuffed into a bin – to a 14-year-old girl in the neighbourhood.

Ten days later, Dennehy killed again. She stabbed her boss and landlord, Kevin Lee, at the same house in Welland. The 48-year-old had started an affair with Dennehy after employing her, and not long before he was killed reportedly revealed to a friend that she had expressed a desire to "dress me up and rape me".

(Photo: SWNS)

Later the same day Dennehy committed her third murder. It happened at the shared house where she was staying on the south side of Peterborough, an isolated area called Orton Goldhay. Here, she attacked John Chapman – a 56-year-old former Navy man who was living on the first floor of the block of bedsits – stabbing him six times.

Afterwards, Dennehy phoned up Gary "Stretch" Richards – who she knew through her work for Kevin Lee – and sang, "Oops I did it again." Bizarrely, Stretch and another man, Leslie Layton, who lived at the Orton Goldhay house with Dennehy, did not go to the police. Layton, in fact, lied to the police, and Stretch helped her dispose of the three bodies in farmland ditches.

Dennehy and Stretch, now acting as her driver, went on the run. Dennehy told Stretch she wanted to kill again: "I want my fun… I need you to get my fun," she reportedly told him. In Hereford, Stretch stopped the car to allow Dennehy to jump out and repeatedly stab two men at random: 64-year-old Robin Bereza and 56-year-old John Rogers. Somehow, both men survived the knife attacks.

Dennehy and Stretch were caught by police a couple of days later, and in February of 2014 Stretch was found guilty of aiding Dennehy on the two attempted murders, while Leslie Layton was found guilty of perverting the course of justice. Dennehy herself was given a whole life sentence for the triple murder and two attempted murders. She is one of only two living women in the UK who will die in prison. The other is Rose West.

The judge told Dennehy she had a "sadistic lust for blood". If the murders weren't enough proof of this, what Dennehy later told a prison psychologist certainly was: "I killed to see how I would feel, to see if I was as cold as I thought I was. Then it got moreish."

The house in the Orton Goldhay neighbourhood, where the third murder took place. (Photo: Adam Forrest)

It's almost four years since the murders, but some in Peterborough still haven't fully recovered.

Toni-Ann Roberts briefly shared the house with Dennehy in Orton Goldhay where she killed John Chapman. The overlap was short; Roberts left about a week after Dennehy moved in. But she also remembers several weeks of Dennehy coming round for rent money, drinking and socialising with the other tenants.

Roberts hasn't been back to Orton Goldhay since the murder of her old flatmate. "John was a lovely man," she recalls. "He was an alcoholic, but a very sweet, gentle person. He reminded me of Uncle Albert from Only Fools and Horses because he would sit back and tell stories about the Navy. There could be no reason at all for doing what she did to him."

Roberts describes Dennehy as "intimidating", but says she did have the disarming ability to get strangers onside very quickly. "She was sort of weirdly flirtatious with people," says Roberts. "She was very upfront about coming in, wanting to know who you were and what you were all about. She was smart about figuring out who was low in confidence and how to push their buttons. The men did tend to fall at her feet like lapdogs. There was this weird hold over men."

In her manipulation of the opposite sex, Dennehy possessed a chameleon-like quality. This was something crime writer Christopher Berry-Dee, author of Love of Blood – a book about Dennehy's life and crimes – noticed when he wrote to her in prison and received a couple of letters back.

"She changes her colours to suit her surroundings," says Berry-Dee. "For instance, she worked out the landlord wanted her for sex. And the Polish guy – she seems to have quickly charmed him into thinking he'd met a nice girl [...] When she was writing back to me her handwriting was beautiful, the spelling and vocab perfect – a calm and collected persona. There was an attempt to impress me. But I did get the chance to see [Dennehy's] letters to Gary Stretch, and it was completely different, like she was slumming it again. She was completely identifying with the person she was writing to."

Gary "Stretch" Richards (Photo: SWNS)

If this suggests someone of icy cunning, it was not the kind of cunning that ever helped Dennehy lead a successful life. She did not maintain relationships. Not even with her own children. She did not skilfully skirt society's rules to achieve material wealth or comfort. She did not function very well anywhere for very long. Dennehy may have boasted about being cold, but the continual self-harm indicates a tortured mind.

Elie Godsi, a consultant clinical psychologist, is convinced she must have suffered some terrible abuse – probably sexual – relatively early in life. "She's an extremely disturbed young woman, who no doubt has had something horrific happen in her background – I would bet my mortgage on it," says Godsi.

"She is violent and sexually violent – that doesn't happen in a vacuum. I have no idea whether it was in the family or not, whether it happened as a child or a teenager, but women do not end up like this without a history," she adds. "Victims become perpetrators because feeling powerful and in control is the antidote to being powerless and controlled. Usually women feel their distress, whereas typically men act on their distress. But not in this case."

Dr Elizabeth Yardley is not so sure. "I've met psychopaths who have been through horrific abuse or neglect as a child and have gone into robot mood to survive," she explains. "But I've met others who've had normal upbringings, been socialised in a seemingly normal way, and have still done horrific things without any conscience."

Could neuroscience tell us more than family history? There is now a small body of research suggesting the hardwiring of psychopaths might be distinct, since a lack of empathy actually shows up on MRI scans. This is intriguing, but not exactly enlightening – activity or a lack of activity in one part of the brain doesn't appear to tell us much about the psyche, or about anyone's readiness to act on unsympathetic thoughts. It seems more likely, as Yardley suggests, that psychopathic violence must be "a messy combination of nature and nurture".

"In Dennehy's case you have the lack of empathy, then you have an escalation of aggressive behaviour over time as boundary after boundary is pushed," she says. "We can try to get closer to understanding, but I'm not sure we'll ever understand 100 percent where the trigger for these kind of aimless murders comes from."

Two of Dennehy's victims: (left) Lukasz Slaboszewski and (right) John Chapman (Photo: SWNS)

Aside from the victims' families, many in Peterborough pulled into Dennehy's orbit have been left with a lingering anxiety, says Toni-Ann Roberts. Part of the uneasiness is a failure for any of it to make sense. "It was so horrific," she says. "I've struggled to understand it. It's affected a lot of us who knew people in that house – it's just not an easy thing to forget about. I'm not the only one who has moved away from Orton Goldhay."

"I can still remember this look she could give you sometimes that was… scary," she says. "The strange thing is, she was like a whirlwind. She came and went so quickly, and left all this terrible stuff behind. She disrupted so many lives."

An elderly man at Orton Goldhay's bus stop tells me something I've heard before in places where people have committed terrible crimes: "It's such a quiet area – you just couldn't have imagined it happening here."

Human beings can be truly terrifying. We know it, but it still shocks us when they do such terrible things. The horrors inside someone's head can spill out into the world, in an ordinary street on an ordinary day, without any recognisable pattern or logic.