Joanna Dennehy
Dennehy bragged about the murders saying she wanted to kill men to become famous.

Joanna Dennehy ripped up the criminal profile by being a female serial killer but experts may never find out what made her carry out her bloody rampage.

When police brought 31-year-old Dennehy's trail of violence to an end by arresting her in Hereford last year, three men lay dead by her hand and two others had only just survived frenzied knife attacks.

What makes the killings - in and around Peterborough - of Lukasz Slaboszewski, 31, Kevin Lee, 48 and John Chapman, 56, so unusual was that all were killed by one woman – a mother of two children.

As a result, Dennehy has joined a rare group of female serial killers, of which there is a very small number of comparable cases in the history of British crime.

Only 6.1% of killers convicted in the UK between 2002 and 2012 were women – and next to none of them were serial killers.

But it could be that Dennehy never opens up about why she killed the men.

Experts warn she could derive twisted pleasure from denying her victims' families the full story of why they died.

That is partly because Dennehy is a diagnosed psychopath, with a host of other mental disorders.

Following her arrest in Hereford, Dennehy was diagnosed with 'a psychopathic disorder'.

Experts also found she had emotionally unstable personality disorder and anti-social personality disorder.

Dr Elizabeth Yardley, senior lecturer at Birmingham City University's centre for applied criminology said Dennehy enjoyed playing games, just like psychopaths who lust for control.

This behaviour may have been on display at her trial when she flummoxed her legal team by suddenly pleading guilty to the murders. Such an act of control is like that exercised by other psychopaths such as Moors killer Ian Brady.

Dr Yardley told the BBC: "This is a woman who clearly likes messing with people, so to throw her own legal team into chaos would have given her a sense of satisfaction."

"But if it had gone to trial and she had given evidence, we wouldn't necessarily be any closer to knowing the truth of why she did what she did," Yardley added.

"Serial killers do not give the truth away very easily - it gives them power that they can continue to use long after they are convicted."

Joanna Dennehy Murder Victims
The bodies of Lukasz Slaboszewski, Kevin Lee and John Chapman were found in Peterborough

Dennehy also told bizarre and delusional lies - another signifier of psychopathy. One of her alleged accomplices claimed she owned up to killing her own father and had gone to prison for it. In fact, her dad was alive.

Dennehy was also found to have a attraction to sexual violence and S&M bondage - a disorder called 'paraphilia sadomasochism'.

This mix of four mental disorders meant she was a 'powder keg' of malevolent pent-up aggression, according to one expert.

Previously, she had taken this dark emotion out on herself by self-harming. There was no indication in her criminal record to suggest she would turn in to a serial killer.

But then something made her turn on other people – to her victims' fatal cost.

Consultant clinical psychologist, Elie Godsi said Dennehy killed "for fun" and "turned the tables" of her behaviour on to her victims.

"Robbery was not a motive," he said. "She was driving around and carrying out random but deliberate attacks."

"She knifed herself, and then other people. Dennehy has a serious history of self-harm. Why was she self-harming? In the case of a psychopath, it is almost always because they have a history of abuse. We are very likely to have here a woman with a history of serious sexual abuse.

"She was driving around and carrying out random but deliberate attacks"
- Elie Godsi, psychologist

"She will be somebody whose experience of sexual relationships is about being brutalised.

"When that happens to young men, they very quickly become violent. It tends to take women much more time to get to the tipping point. It is much more likely for them to self harm and have mental health problems.

"Men act out their distress, women feel their distress," he told the BBC.

Joanna Dennehy admitted to the murders at the Old Bailey (Cambridgeshire Police)
Joanna Dennehy admitted to the murders at the Old Bailey (Cambridgeshire Police)

Another insight in to Dennehy's dark mind came after she stabbed her final victim and then phoned a friend and sang Britney Spears' pop song Oops I Did It Again down the line. It displayed a callous disregard for other people which is one of the characteristics of psychopathy.

Women's status in society makes Dennehy's own crimes more shocking, said Godsi.

"It shakes to the very core our stereotypes and our cultural frameworks about womanhood and femininity," he said.

"As far as we are concerned women don't do these sorts of things. Women care, women treat and look after and empathise."

Not Dennehy. It remains to be seen whether her name now becomes as infamous as fellow female serial killers such as Rosemary West and Myra Hindley, but she does belong in the same bracket.