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It was the chilling moment John Treanor realised he had to get as far away as possible from dangerwoman Joanna Dennehy.

He shuddered as he recalled how she suddenly pulled a six-inch dagger from inside her black knee-length boot.

Then, tightly gripping the decorated handle of the blade she thrust it into the carpet, her eyes cold and blank.

John, the father of her two children, looked on in fear and horror.

The young woman he had fallen in love with was now unrecognisable – but not even he could predict she would go on to become a serial killer.

Joanna, who had dreamed of becoming a lawyer, was now ravaged by drink and drugs, her body scarred from dozens of self-inflicted knife wounds.

Within months John had fled their home to start a new life with his daughters, miles away from their mum.

Four years later the police knocked on his door to say his ex was suspected of murdering three men.

And on Monday Dennehy, 31, appeared at the Old Bailey and pleaded guilty to stabbing them to death and dumping their bodies in ditches.

She also admitted the attempted murder of two other men.

Her appearance in the dock marked the astonishing downturn her life had taken since her days as a pretty teenager when her future seemed bright.

Dennehy’s parents Kevin, 56, and Kathleen, 51, who have another daughter, Maria, 29, had paid for extra lessons to help her achieve her legal ambitions.

But it wasn’t long before the cracks began to show. She was sent home from school for drinking and John, now 37, said she was already rebelling against her home life when they met.

Sitting in his home, newly married and surrounded by photos of his two children, he can still hardly believe how much Dennehy changed during the 12 years they spent together.

They met while he was walking his German Shepherd in a park in 1997.

He said: “She approached me. She had a thing for dogs – it just went from there. She’d fallen out with her parents and she was a bit of a free spirit but I liked her – in fact I loved her.”

Despite the six-year age gap, they became inseparable, to the horror of her parents who kicked her out of their £315,000 four-bedroom home in St Albans, Herts.

John explained: “She had been in trouble at school for drinking, she was also stealing. At 15 her mum kicked her out and told her not to come back.” John’s family also refused them a roof because of her age and the pair began sleeping rough for a year. John, 37, a non-drinker, admitted he was no angel and had used cannabis in the past.

He said: “I was a bit of an a******e when I was younger. I got into trouble a lot. It was just stealing from shops, that sort of thing.”

But as a life-long teetotaller he said it was booze and not their relationship that brought about ­Dennehy’s drastic personality change.

The pair found digs at a shared house in Luton, Beds, but had to flee to Milton Keynes, Bucks, after they tipped off police about a drug dealer in the house.

John insisted he never slept with Dennehy until she was 16.

They had their first child in 1999, when she was 17. He was delighted but she was ­devastated. He said: “She never wanted the kids, she always said she wanted it to be just me and her. We took a photo of her, holding the baby. You could see in her face, she was not interested.” John said it was the start of her decline into drink, drugs, violence and casual sex with both men and women.

He added: “She cheated on me with a neighbour and she was using cocaine. I was out working 17 hours as a security guard she was drinking at friends’ houses till god knows what time.”

Betrayed by her affair, John left with his daughter, moving to King’s Lynn, Norfolk, where he took her back.

She got a job, working on the nearby farms digging vegetables, but her drinking escalated.

He said: “It got to the point where she was not being paid in cash, she was being paid in bottles of whisky and vodka. She was coming home ­paralytic, smashing the place up.”

On one occasion, she almost pushed her three-year-old daughter down the stairs, barging past her in a drunken rage.

John said: “She was absolutely wasted. I got a couple of punches. That day I left with my daughter and went back to my mum’s.”

(Image: PA)

For 18 months he had no contact with Dennehy and was told she had spent time in a psychiatric unit and prison. He also heard rumours of prostitution.

But she made contact once again and, against his better judgment he took her back.

They set up home in 2003 in Wisbech, Cambs. Dennehy reined back her boozing and three years later the couple had a second daughter.

The new arrival sent her back to the bottle – John said she started drinking strong lager for breakfast.

He added: “I tried everything to get her off booze and drugs and show her what she had sitting right in front of her. But she wouldn’t even give the baby a cuddle or a kiss.”

Dennehy’s friendship with a woman called Charmaine drove another rift between them. John heard the pair were lovers. He said: “She was coming home with love-bites on her neck. She’d ­disappear for weeks, come back and ask for forgiveness, then go away again. I was a complete walkover.”

Dennehy had self-harmed previously but now the wounds became more noticeable. He explained: “She was slicing herself with razor blades, all up her arms and around her neck. She was cutting everywhere.”

Dennehy was drinking up to two litres of vodka when the incident with the dagger happened in early 2009.

John said: “It scared the life out me.”

Soon afterwards he took his two daughters and moved to his mother’s home in Glossop, Derbys.

He revealed that his eldest daughter, now 14, has started asking questions about her mum and wants to visit her in jail.

But John has no sympathy for his former lover.

He said: “It was inevitable this was going to happen. She was either going to do something to herself or to someone else.

“She deserves what is coming to her and if the death penalty was still here she would deserve that.”