A serial killer met his third victim while on unescorted leave from a secure mental hospital where he had been locked up for strangling his last girlfriend, a court heard.
Violent and "controlling" Theodore Johnson, 64, attacked mother-of-four and grandmother Angela Best, 51, beating her over the head with a claw hammer and throttling her with a dressing gown belt.
He then jumped in front of an express train at Cheshunt station in Hertfordshire.
As he was being treated for severe injuries, police went to his flat and found Ms Best dead in the living room.
Johnson, who is now confined to a wheelchair, pleaded guilty at the Old Bailey to murdering Ms Best and appeared before Judge Richard Marks QC to be sentenced.
Prosecutor Mark Heywood QC said Johnson had a violent history towards the women in his life, having been convicted of manslaughter twice before.
In 1981, he was convicted of killing his wife by pushing her off a balcony and then, in 1993, a couple of years before meeting Ms Best, he was convicted of strangling another girlfriend in London.
The court heard that Ms Best, of Tottenham, north London, had been in a relationship with Johnson since around 1996, after moving to the capital from Manchester.
At the time, Johnson was still the subject of a hospital order, having been refused conditional discharge around that time.
The Jamaican, who suffered from a personality disorder and depression, was on unescorted leave when he met Ms Best as he was taking a City and Guides course on furniture restoration on two days a week.
In October 1997, about a year after their relationship began, Johnson successfully applied to a tribunal for a conditional discharge.
One of the conditions was that he alerted authorities to any relationship with a woman, which he failed to do for years.
He kept his relationship a secret from doctors and social workers who continued to monitor him in the community.
He was last seen by a social worker and psychiatrist on December 8 2016 - days before the murder - and was not found to be depressed and continued to deny being in a relationship.
On the condition of his release, Mr Heywood said: "The tribunal recorded Mr Johnson was well aware of the need for extreme caution with regard to any further relationship with women and understands Condition 4 and the purpose of it and its consequences.
"That decision having taken place on October 30 1997, by then Mr Johnson had effectively been in a relationship with Ms Best for about a year or so during his unescorted release from secure accommodation."
During their relationship, Johnson had been abusive and "controlling" and had punched her on more than one occasion, according to her family.
Ms Best only found out he had killed a previous partner when she found letters and confronted him, the court heard.
In September 2016, the couple split up and Ms Best was said to be the "happiest" she had ever been after finding love with someone else.
But Johnson continued to profess his "undying love daily".
On the morning of December 15 2016, Ms Best had gone to Johnson's home to help with an appointment with the Jamaican embassy.
Mr Heywood said: "Soon afterwards he attacked her. That attack was brutal and merciless. He used a claw hammer. He struck her repeatedly around the head even as she tried to protect her head with her hands. He then tied a dressing gown cord around her head and knotted it. She at that time was unable to defend herself.
"He did it, the prosecution say, for a simple reason - because after all that time that had gone before she was no longer prepared to stay with him."
After killing Ms Best, Johnson was seen to topple forward into the path on the oncoming 3.18pm express service, severing both arms.
When officers when to his flat in Dartmouth Park Hill in Islington, north London, Ms Best's body was discovered on the floor of the living room, near a bloodstained claw hammer.
A post-mortem examination found she had suffered at least six blows to the head with the hammer and been strangled.
Mr Heywood said Johnson was born in Jamaica and came to Britain in 1980 and worked at a car repairer's.
In November 1981, he was convicted by a jury at Stafford Crown Court of the manslaughter of his wife, Yvonne Johnson, by reason of "provocation".
Following an argument, he had picked up an ashtray and glass vase and hit the mother-of-two before pushing her over the balcony of their ninth-floor flat in Wolverhampton.
Then in March 1993, he was convicted at the Old Bailey of the manslaughter of his partner, Yvonne Bennett, by diminished responsibility.
The couple, who had a daughter together, had moved from Wolverhampton to London, where Ms Bennett had an affair with another man.
She had sought police help to get him to leave their Finsbury Park home in the days before he strangled her with a belt and then tried to hang himself.
Campaigners have called for an investigation into how he was dealt with by the justice system and whether any of the deaths could have been avoided with better monitoring by agencies in the community.
Camden and Islington NHS Trust, which was responsible for Johnson's care in the community since 2004, said it would provide Ms Best's family with an independent report and that Johnson's treatment complied with conditions set by the Mental Health Tribunal which oversaw his discharge in 1997.
Ms Best's sister, Valerie Archibold, said preparations for a joyful Christmas were destroyed when the family learned of her "brutal and heinous murder".
She described Ms Best as a generous and loyal person and the "life and soul of our family".
The impact of her death was "immeasurable", she said. "The shock still remains and the loss is traumatising, immense, profound and utterly devastating."
Mitigating for Johnson, Annette Henry QC said: "He does not wish to be alive. He hates himself for what happened. We recognise the devastation felt by the family members."
Ms Henry added that Johnson was likely to die in prison.
The court heard that Johnson had an appointment two days before Ms Best's murder but it was put off because his social worker was sick.
On his condition following his release from the psychiatric hospital, Mr Heywood said: "He was examined for signs of depressive illness. No signs of depressive illness were seen at any stage in relation to these events."
Ms Henry said the mental health tribunal's restriction on Johnson was flawed as it relied on "self reporting" any new relationship.
She said: "This was a dilemma and the tribunal found it was fraught with difficulty in trying to monitor."
She pointed out that on visits to his home, authorities saw "a feminine wood carving and letters of love on his mantelpiece" but did not pick up on the signs.
Judge Marks said: "This was a deception that must have gone on for something like 15 years."
In his victim impact statement, son Raphael Best said: "My mother was the type of person who was always going out of her way to help people and unfortunately that was the reason she met her demise."
He said the loss of his mother was "slowly driving me crazy", adding: "It makes me feel 10 times worse when I think of the kind person my mother was."
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