Sarah Williams, 35, shot successful businesswoman Sadie Hartley, 60, with a 500,000-volt stun gun as her unsuspecting victim answered the door at her home, Preston Crown Court heard.
Seconds later, with “demonic savagery”, she used a kitchen knife to stab the semi-paralysed mother-of-two in the face and neck, inflicting more than 50 injuries, before leaving her in a pool of blood in the hallway of the £500,000 house in Helmshore, Lancashire.
Ms Hartley was home alone because her partner, Ian Johnston, was away on a skiing trip in the Swiss Alps.
Williams, described as a “bunny boiler” and “kept woman” who was already in a relationship with a wealthy 75-year-old “sugar daddy” and having affairs with other men, had a brief fling with ex-fireman Mr Johnston.
He had dumped “obsessive and jealous” Williams and begun a new life with Ms Hartley - but explicit photos and sex texts continued between the pair while Williams plotted the “perfect murder” for 18 months to kill her rival and win him back.
Today she was convicted of the murder on January 14, alongside her accomplice, horse riding instructor, Katrina “Kitt” Walsh, 56, following a seven-week trial.
Both women, from Chester, had denied murder and blamed each other.
The jury returned to court after seven hours and nine minutes of deliberations.
Williams swallowed hard as the forewoman delivered the “guilty” verdict, while Walsh gave a slight nod of her head.
Ms Hartley’s daughter, Charlotte, 23, wiped away tears as she sat in the public gallery next to Garry Hartley, her father and Sadie’s ex-husband, and her brother, Harry.
Mr Johnston, sitting a few rows behind them, had tears in his eyes and gave a slight nod as the verdicts came in.
Mr Justice Turner thanked the jury for sitting on the seven-week trial and told them he will sentence both defendants at 2.15pm today.
Both women were then taken down to the holding cells.
Outside court, Detective Superintendent Paul Withers, who led the investigation for Lancashire Police, said: “This murder was nothing short of the cold-blooded, premeditated and carefully planned assassination of an entirely innocent woman and I welcome today’s verdicts.
“I would like to offer my thanks to the jury for their careful consideration of all of the facts in this case and the prosecution team for all of their hard work in bringing this case to court and to a successful conclusion.
“While it may have been Sarah Williams who carried out the actual killing of Sadie Hartley, there can be no doubt that Katrina Walsh helped her every step of the way and was up to her neck in the planning of this brutal slaying. They are both as culpable as each other.
“I’d like to pay tribute to Sadie Hartley’s family, who have conducted themselves with dignity throughout this trial and following the unimaginable horror of what happened to their loved one. I can only hope that today’s verdicts offer them a chance to start to move on with the rest of their lives and our thoughts remain with them.
“I would also like to pay tribute and to thank all of the officers and staff who worked so hard to investigate this dreadful crime and to bring these two women to justice. An incredible amount of dedication, professionalism has gone in to helping secure today’s convictions.
“Finally, I would like to thank the community of Helmshore, especially those in Sunnybank Road, for their understanding and support during the early stages of this investigation. Their co-operation was vital.”
Williams, who described herself as a “She Devil” and “little psycho”, had sent Ms Hartley a “spiteful” letter in June 2014, claiming she and Mr Johnston had enjoyed “unbelievably fantastic sex ... over and over and over again” behind her back, but it failed to end the relationship of the couple, who were both divorcees with grown-up children.
She bombarded him with explicit photos and texts and Mr Johnston, “flattered” by the attention of a younger woman, said in a moment of “weakness” he replied in kind.
A month before the murder in one text exchange with Williams about spanking, he replied: “What man wouldn’t this work for? Dangerous I know.”
During seven weeks of evidence the jury heard that the plot had unfolded for 18 months, with Williams and Walsh travelling to Germany last December to buy the stun gun before purchasing a £430 second-hand Renault Clio and dark clothing to use in the “murder mission”.
Williams, who worked as a ski holiday sales rep, had secretly attached an electronic tracker to Mr Johnston’s car to follow his movements, determined to win him back.
Along with Walsh, she drove to Helmshore to check they had the right address, where Walsh handed over flowers to a surprised and disturbed Ms Hartley, saying “These are for you, Ms Hartley”.
A week later, almost exactly to the minute, Williams returned alone, dressed in dark clothing, armed with the stun gun and 8in (20cm) carving knife, killing her victim in an “orgy of violence”.
CCTV caught her arriving 40 yards from her victim’s home, going to the house and returning back on camera four minutes 40 seconds later, a “no-hesitation, determined and swiftly executed murder”.
Ms Hartley’s body was discovered the next day by police, when worried friends and family failed to contact her.
The victim, a keen horse rider who ran her own medical communications business, had been due to fly out to Switzerland to be with Mr Johnston two days later.
Earlier the court heard that Williams first met Mr Johnston at the Chill Factore dry ski slope in Manchester in 2012.
Mr Johnston, an accomplished telemark skier, gave her lessons and the pair exchanged phone numbers and began texting each other, which quickly became flirty.
He told the jury Williams then turned up at his house wearing a short skirt, high heels and red lipstick and the relationship became sexual.
Williams had been seeing wealthy, married oil firm owner David Hardwick, 75, since she was 17 and he was 57, and the two enjoyed dozens of exotic holidays a year. He gave her £75,000 to buy a house and put £320 a week in her bank account.
Mr Hardwick was not aware of her liaisons with Mr Johnston or another lover she had a relationship with - married Thai martial arts expert Somapat Sitiwatjana, known as Master A.
Williams also sent his wife a tell-all letter, before moving on to another affair with another ski-instructor, Andy Poole, before the relationship began with Mr Johnston.
Text messages continued even as Mr Johnston and Ms Hartley grew closer and eventually began living together, the businesswoman buying their four-bedroom, £500,000 detached home.
But while Williams and Mr Hardwick were away on ski trips with Ms Hartley and Mr Johnston, the former lovers played “footsie” under the table and sent each other sex texts.
Williams behaved like a “besotted teenager” with Mr Johnston and told friends Ms Hartley was a “bitch troll” who had “trapped” her partner through her money.
But Mr Johnston told the jury the relationship was “just sex” and “electronic flirting”.
He denied leading her on but admitted swapping sex texts until just days before Williams murdered his partner.
Detectives arrested Williams within three days of the murder and seized Walsh’s diaries.
The “extraordinarily revealing document” showed how the pair had planned the “assassination”.
In June last year Walsh wrote: “Seriously talking of getting rid of her opponent ... she does seem to be a totally evil bitch.”
Later she wrote: “I have no moral qualms, just a serious don’t let us get caught twinge”, and in another entry “Took ages to wind down, all the excitement of plotting the perfect murder!”
Walsh later handed over the diaries to police and took them to Collinge Farm in Chester, where her horse was stabled, revealing where she had buried the stun gun and knife.
Williams suggested Walsh was responsible and the pair ran a “cut-throat” defence, each blaming the other.
Williams had been sent home from work ill and was in bed reading Game Of Thrones on the night of the killing, she claimed.
But detectives trawled CCTV and tracked the cars used by the pair to reveal their movements in the run-up to the murder.
Forensic officers also discovered Ms Hartley’s DNA in Williams’ bath, probably after she had showered her victim’s blood off her, and on her spectacles in her Volvo car - for which the defendant had no explanation.
Walsh did not give evidence, her barrister telling the jury that, while she may be a “vile” individual, she never believed Williams would kill.
She went on to tell police that she “largely thought” she was taking part in a game of the Channel 4 programme Hunted, where teams try to evade detection from military-style trackers and surveillance.