Joanna Dennehy could be charming, a good friend, and attractive to both men and women. But if she did not get her way, she was prone to explosive, frightening outbursts of temper and violence. She would punch and lash out at those closest to her and friends and lovers got used to getting out of the way quickly.

Over a fortnight in early spring last year, Dennehy went much, much further, killing three men, possibly only because she had been annoyed by their unwanted attention or grown bored by them after casual sexual liaisons. Having tasted and apparently been thrilled by violence, she went in search of more and only narrowly failing to murder two other men before she was caught.

Doctors who have examined Dennehy before and after her killing spree have diagnosed a range of conditions. A year before she struck she was found to have a psychopathic anti-social personality disorder that manifested itself in anger, aggression, impulsivity and irresponsibility. She did not care about the safety of others, felt no remorse when causing harm, and was a skilled deceiver and manipulator. Like many who have a severe anti-social personality disorder, Dennehy has frequently been in trouble with the law and has a long history of serious drug and alcohol abuse. Her sexual preferences disturbed some of her many partners – men and women – but added to her allure for others. After her arrest she was diagnosed as having paraphilia sadomasochism: deriving sexual pleasure from both giving and receiving pain and humiliation. She did not hide her tendencies, often wearing a pair of handcuffs attached to her trousers. She was a consumer of violent pornography and an habitual self-harmer, cutting her body, sometimes even during intercourse.

Police picture of serial killer Joanna Dennehy. Photograph: Cambridgeshire Police/PA

All that said, her murderous spree in the spring of last year was a huge shock to her friends, her very respectable home counties family and to police. Though she had significant mental health problems and a criminal record, there was nothing to suggest she should suddenly murder three times and try to kill twice more.

Born in August 1982 in St Albans, Hertfordshire, Dennehy grew up in nearby Harpenden. Her father, Kevin, worked as a security guard, while her mother, Kathleen, was a shop manager. She has a younger sister called Maria, who had a successful career in the army and went on to run her own IT company. Denneny has claimed that she was abused as a child but there is no evidence of this, and her family and friends insist she was always treated well.

Her childhood sounds comfortable and ordinary. As a little girl she shared a bunk bed with Maria, who was two years her junior, and they were so close that they invented their own secret language. Dennehy was fond of her dolls and as she grew up loved make-up, doing her hair and fashionable clothes. Friends say their parents were strict with the girls and protective but never overbearing.

At first Dennehy did well at Roundwood Park School in Harpenden. Bright and capable, her parents hoped she would go on to university and become a lawyer and even paid for extra tuition for her. She played for school hockey and netball teams.

Gary 'Stretch' Richards, 47, (left) and Leslie Layton, 36, who aided serial killer Joanna Dennehy and have been convicted of further offences relating to her killing spree. Photograph: Cambridgeshire Police/PA

It all began to go badly wrong in her mid-teens. She started skipping lessons and associating with a crowd of older boys. When she was about 15 she fell for a man called John Treanor, who was five years her senior, and ran away with him. Her family were frantic. The pair were eventually found living rough on wasteland close to the family home and there was a reconciliation.

But by this time Dennehy had begun taking drugs and drinking, even turning up to school high or drunk. Aged 16 she left home for good, only returning occasionally when she needed money.

Dennehy and Treanor set up home first a few miles up the road in Luton, then in Milton Keynes. While she was still in her teens, she had two children. She became permanently estranged from her parents following the birth of her first child when she told her parents that they would have pay to see their grandchild.

The relationship with Treanor was stormy. She cheated on him with men and women. She was a poor mother, telling friends she had never wanted children. Dennehy would leave home for days or weeks on end, then return and ask for forgiveness.

Police picture of Joanna Dennehy at Strensham Service station off the M5 with Gary 'Stretch' Richards. Photograph: Cambridgeshire Police/PA

They tried a fresh start, moving to East Anglia, but her drinking got worse. She worked as a labourer on farms and would sometimes be paid in alcohol rather than cash. She harmed herself, cutting her arms, body and neck with razor blades and etched a tattoo of a star under her right eye herself.

While Joanna's decline accelerated, her sister Maria was serving with British troops in Afghanistan. After her tour of Helmand, she traced her sister and Treanor to an address in Cambridgeshire. But Dennehy made it clear she wanted nothing to do with her family.

Her violent tantrums became worse. She would kick and punch Treanor when she was drunk. She began to carry a dagger in one of her boots and told friends she often felt like killing someone. Afraid that she would use the dagger on him, Treanor left in 2009, taking the children with him.

Dennehy's behaviour went from bad to worse. She drifted from address to address in East Anglia, stealing and sometimes turning to prostitution to fund her drug and drink habit. She served time in prison and received some treatment for her mental health problems.

In February 2012 – just over a year before the murders – Dennehy spent a few days in Peterborough city hospital, where she was diagnosed as having an anti-social personality disorder and obsessive compulsive disorder. After her arrest she was examined by a consultant forensic psychiatrist, Frank Farnham, while she was being held at Bronzefield prison in Surrey. He said she had paraphilia sadomasochism. Those with the condition experience sexual excitement from acts involving the infliction of pain, humiliation or bondage. Dennehy liked to give – and receive – pain.

It is also clear that she revelled in her notoriety, jumping for joy when police appealed for help finding her and describing herself as a "monster" while on the run. She boasted that she had killed eight men in all – including her father, two people in a house fire and two in a hit-and-run. Her father is very much alive and no evidence has been produced so far that she has murdered more than three men.

Strictly speaking, she is not a serial killer – criminologists say there has to be a "cooling off" period between murders and would classify her as a "multiple murderer or "spree killer". She will join the likes of Rosemary West and Myra Hindley as one of Britain's most infamous female murderers but detectives and academics see her as a one-off.

"This case is unique and unprecedented," said David Wilson, professor of criminology at Birmingham City University. "Serial killers disengage from the process of killing and revert to their normal life. There was never any sense of her disengaging. She seems to have constantly been in the moment of killing." Wilson, who has studied the case closely, said this could have been due to a number of factors, including her use of drugs and alcohol, and the relationship she had with Gary Stretch, who helped her dispose of her victims' bodies and went on the run with her.

Wilson suggests this created a syndrome called folie à deux, or shared psychosis. "It creates a world in which even the most extraordinary ideas and behaviours are seen as being permissible. So often it is the woman who follows the world view of the more dominant male partner. In this case Dennehy was the dominant partner and Stretch accommodated her world view."