Joanna Dennehy killed for fun. She was diagnosed as suffering from paraphilia sadomasochism while she was being held at Bronzefield Prison in Surrey.
The jury in the trial of her two alleged accomplices heard Dennehy, who admitted murdering three men and trying to kill two others, gained sexual excitement from acts involving the infliction of pain.
The diagnosis by the consultant forensic psychiatrist holds the key to understanding Dennehy's crimes.
Dennehy spent five days in a psychiatric ward at Peterborough hospital in February 2012. She was diagnosed as having a personality disorder and psychopathic disorder.
Paraphilia is a condition characterised by abnormal sexual desires. The jury was told that some people prefer to be the recipient of pain in sadomasochistic acts while others preferred to provide it.
In the space of just 10 days last spring she murdered three men and then attempted to murder two others.
Lukasz Slaboszewski, 31, a man she had just met, was the first victim. John Chapman, 56, a housemate, followed ten days later.
Then just hours later Kevin Lee, 48, her landlord, who she was having an affair with, was murdered.
All three men were stabbed in the heart. Their bodies were then dumped in ditches in Cambridgeshire.
John Chapman was a Falklands veteran and rented a room in the house where Dennehy was also a tenant.
Kevin Lee was a father of two. His wife feared his life was at risk after she learnt what was going on. He was stabbed to death just hours after John Chapman.
Dennehy also pleaded guilty to the attempted murders of Robin Bereza and John Rogers, whom she randomly selected and stabbed in the street in Hereford. They were left for dead but survived.
"If we look at murder as a phenomenon, Dennehy is unprecedented in British legal history," criminologist Professor David Wilson from Birmingham City University told ITV News.
Technically a serial killer has a fixed definition in this country as someone who kills three or more people in a time period of greater than 30 days. The victims of a serial killer are not all killed at once, but over a much longer time period - usually with a cooling off period between each victim. In Dennehy's case that 'cooling off' period never seems to happen.
Dennehy, the court heard, was 'ecstatic' when she saw news reports that she was wanted by police. She likened herself and Gary Stretch to 'Bonnie and Clyde'. A friend told the court when Dennehy heard her name mentioned on the news she "jumped around like a schoolgirl".
As Dennehy was looking for more victims with Gary Stretch in Hereford she said: "I want my fun. I need you to get my fun."
Detective Chief Inspector Martin Brunning, from Bedfordshire, Cambridgeshire and Hertfordshire Major Crime Unit, is in no doubt of the danger Joanna Dennehy poses.
"In the way that she behaves and the apparent unemotional way she goes about her terrible attacks this makes her absolutely the most dangerous person I have ever met and a huge danger to the public for the foreseeable future," he told ITV News.