Tetsuyuki Morikawa was born on April 10, 1930, in Tenmei, Kumamoto Prefecture. He was the third son of a fisherman. The area where he was born had rough land, which led his father to make a living by fishing for seafood and seaweed. When Tetsuyuki was only two years old, his father was killed during a fight with another fisherman. After this tragedy, Tetsuyuki's mother remarried and left him and his older brother. The brothers were sent to live with their paternal grandparents.

When Tetsuyuki was ten, his grandmother died. As a result, he moved in with his paternal aunt and uncle. They were a wealthy couple without children and welcomed Tetsuyuki and his brother. However, Tetsuyuki struggled to adjust. He dropped out of Zendomo Elementary School after just one year. By age fifteen, he was arrested for assault and fined. He continued to get into trouble and was arrested again later that year for another assault and embezzlement.

After he became an adult and was released from prison, Tetsuyuki worked as a construction worker. He traveled between different prefectures, such as Fukuoka, Hyōgo, Mie, and Aichi, but often quit due to injuries. He faced further legal issues and ended up in jail again for another assault.

In 1958, Morikawa returned to his hometown of Kumamoto. He began dating a woman known only by the alias 'X.' They married in January 1959 and moved into a new home built by his in-laws. The couple had two sons within a year. At first, Morikawa supported his family by selling seaweed. However, he soon developed a gambling addiction. He refused to work and spent much of his time drinking shōchū, a type of Japanese liquor.

During this time, Morikawa's behavior grew increasingly abusive toward his wife. He began pawning her belongings and physically attacking her. His wife sought help from Morikawa's relatives but continued to endure excessive abuse. Eventually, she could no longer tolerate it and divorced him in 1962, leaving their children with Morikawa's brother and his wife. Despite the divorce, Morikawa found her and forced her to return to Kumamoto, seeking to reconcile with her.

On September 15, 1962, Tetsuyuki Morikawa gathered with family members at his aunt and uncle's house in Kumamoto to discuss his behavior towards his wife. Morikawa pretended to leave to get some money but instead bought a knife at a hardware store. Later that evening, he confronted his wife and mother-in-law at a bus stop. During the confrontation, his wife announced that she intended to break up with him. In response, Morikawa stabbed her twice and then attacked her mother, stabbing her multiple times as well. His mother-in-law did not survive the attack, but his wife was taken to the hospital and survived her injuries. Morikawa was arrested soon after, charged with murder and attempted murder. He was found guilty and sentenced to life imprisonment.

Morikawa was granted parole on December 8, 1976. He moved back in with his aunt and uncle but refused to get a job and engaged in excessive drinking. Tensions rose, leading to arguments and physical confrontations. On June 21, 1978, he attempted to stab his aunt and uncle during one such argument but they escaped and contacted the police, resulting in his arrest and return to prison. During this time, he developed resentment towards those he felt had wronged him.

After another parole in February 1984, Morikawa lived in a probation facility in Kitakyushu. While there, he created a detailed murder plan targeting his ex-wife and her relatives. He was able to escape on May 31, 1985, and convinced his brother that he had changed. He attempted to locate his ex-wife but was unsuccessful, so he decided to target other relatives. On July 22, 1985, he bought a sashimi knife and spent the night near his intended victim’s house.

The following day, Morikawa returned to the house of a 65-year-old woman, A, and her adopted daughter, B. After waiting for them to sleep, he broke in. He threatened A at knifepoint and stabbed her multiple times. B woke to the commotion and was also attacked. Morikawa stabbed B several times as she tried to escape, killing both women.

Afterward, Morikawa attempted to hide his actions by stripping the bodies and washing his clothes. He stole money and valuables before leaving the scene. The bodies were discovered on July 24, 1985, when employees at A's workplace became concerned over her absence. The police quickly connected Morikawa to the crime through fingerprints found at the scene.

On July 28, 1985, Morikawa was apprehended at a local racecourse after being recognized by a taxi driver. He admitted to the murders during questioning and was charged with multiple offenses, including robbery and murder.

His trial took place in 1986, where he was found guilty and sentenced to death. The court noted the premeditated nature of the crime and Morikawa's lack of remorse. His appeals were rejected, and he remained on death row.

On September 10, 1999, Tetsuyuki Morikawa was executed by hanging in the Fukuoka Detention House. Other inmates were also executed on the same day, highlighting his case amid ongoing discussions about capital punishment in Japan.