in 1953, Ruth became the manager of the Little Club, a nightclub in Knightsbridge. At this time, she was lavished with expensive gifts by admirers and had a number of celebrity friends. Ellis met David Blakely, three years her junior, through racing driver Mike Hawthorn. Within weeks, he moved into Ruth's flat above the club despite being engaged to another woman, Mary Dawson. Ruth became pregnant for a fourth time but had her second abortion, feeling she could not reciprocate the level of commitment Blakely showed towards their relationship.
Ruth then began seeing Desmond Cussen, a former Royal Air Force pilot who had flown Lancaster bombers during the Second World War, and who had taken up accountancy after leaving the service. He was appointed a director of the family business Cussen & Co., a wholesale and retail tobacconist with outlets in London and South Wales. Ruth eventually moved in with Cussen at 20 Goodward Court, Devonshire Street, north of Oxford Street. The relationship with Blakely continued, however, and became increasingly violent as he and Ruth continued to see other people. Blakely offered to marry Ruth; she consented, but in January 1955 she had a miscarriage after he punched her in the stomach during an argument.
On Easter Sunday, 10 April 1955, Ruth took a taxi from Cussen's home to a second-floor flat at 29 Tanza Road, Hampstead, the home of Anthony and Carole Findlater, where she suspected Blakely might be. As she arrived, Blakely's car drove off, so she paid off the taxi and walked the 1⁄4 mile (400 m) to The Magdala, a pub in South Hill Park where she found Blakely's car parked outside.
At around 9:30 pm, Blakely and his friend Clive Gunnell emerged. Blakely passed Ruth waiting on the pavement when she stepped out of the doorway of Henshaw's, a newsagent next to The Magdala. As Blakely searched for the keys to his car, Ruth took a .38 calibre Smith & Wesson Victory Model revolver from her handbag and fired five shots at Blakely. The first shot missed. Ruth pursued Blakely as he started to run around the car, firing a second shot which caused him to collapse onto the pavement. She then stood over him and fired three more bullets, with one fired less than half an inch from his back, leaving powder burns on his skin.
Ruth was seen to stand over Blakely as she repeatedly tried to fire the revolver's sixth shot, finally firing it into the ground. This bullet ricocheted off the road and injured Gladys Yule, a bystander, who lost the use of her right thumb.
She was arrested immediately by an off-duty policeman, who heard her say, "I am guilty, I'm a little confused." Blakely's body was taken to hospital with multiple fatal wounds to the intestines, liver, lung, aorta and trachea.
At Hampstead police station, Ruth appeared to be calm and not obviously under the influence of drink or drugs. She made her first appearance at a magistrates' court on 11 April 1955, and was ordered to be held on remand. Ruth was twice examined by principal Medical Officer, M. R. Penry Williams, who failed to find evidence of mental illness; an electroencephalograph examination on 3 May found no abnormality. While on remand, Ruth was examined by psychiatrist Duncan Whittaker for the defence and by Alexander Dalzell on behalf of the Home Office. Neither found evidence of insanity.
On 20 June 1955, Ruth appeared in the Number One Court at the Old Bailey, London, before Mr Justice Havers. She was dressed in a black suit and white silk blouse with freshly bleached and coiffured blonde hair. Her defending counsel, Aubrey Melford Stevenson, supported by Sebag Shaw and Peter Rawlinson, expressed concern about her appearance (and dyed blonde hair) but she did not alter it to appear less striking.
The only question put to Ruth by prosecutor Christmas Humphreys was, "When you fired the revolver at close range into the body of David Blakely, what did you intend to do?"; her answer was, "It's obvious when I shot him I intended to kill him." This reply guaranteed a guilty verdict and the mandatory death sentence. The jury took twenty minutes to convict her.
Ruth remained at Holloway Prison while awaiting execution. She told her mother that she did not want a petition to reprieve her from the death sentence and took no part in the campaign. However, at her relatives' urging her solicitor, John Bickford, wrote a seven-page letter to Home Secretary Gwilym Lloyd George setting out the grounds for reprieve. Lloyd George denied the request.
On 12 July 1955, the day before her execution, Mishcon and Simmons saw Ruth, who wanted to make her will. When they pressed Ellis for the full story, she asked them to promise not to use what she said to try to secure a reprieve; Mishcon refused. Ruth divulged that Cussen had given her the gun and taught her how to use it on the weekend prior to the murder. She also revealed that Cussen had also driven her to the murder scene. Following a two-hour interview, Mishcon and Simmons went to the Home Office; the Permanent Secretary, Sir Frank Newsam, was summoned back to London and ordered the head of Criminal Investigation Department (CID) to check the story. Lloyd George later said that the police were able to make considerable enquiries but that it made no difference to his decision, and in fact, made Ruth's guilt greater showing the murder was premeditated. He also said that the injury to the bystander was decisive in his decision: "We cannot have people shooting off firearms in the street!"
In a final letter to Blakely's parents from her prison cell, Ruth wrote, "I have always loved your son, and I shall die still loving him."
The Bishop of Stepney, Joost de Blank, visited Ruth prior to her execution. Just before 9 am on 13 July, the hangman Albert Pierrepoint and his assistant entered her cell, and took her to the adjacent execution room where she was hanged. As was customary in British executions, Ruth was buried in an unmarked grave within the walls of Holloway Prison. In the early 1970s, the remains of executed women were exhumed for reburial elsewhere; in Ellis's case, directed by her next of kin, son Andy, her remains were reburied in the churchyard of St Mary's Church in Amersham, Buckinghamshire, some 3 mi (4.8 km) from where Blakely was buried. Her headstone was inscribed "Ruth Hornby 1926–1955". Andy destroyed the headstone shortly before he committed suicide in 1982.