Futoshi Matsunaga was born on April 28, 1961, in Kokurakita Ward of Kitakyūshū, Fukuoka Prefecture, Japan. He grew up in the nearby city of Yanagawa. In school, Matsunaga was known for receiving good grades and had a charming personality. However, he also had disciplinary issues. At one point, he was transferred to another high school because he was involved in a relationship with a junior high school girl.

At the age of 19, Matsunaga got married and became a father to a son. Even though he was married, he had affairs with at least ten other women. In October 1982, Matsunaga began an affair with Junko Ogata, who was a former schoolmate from Yanagawa. In 1984, he promised to marry Junko, but her mother, Shizumi Ogata, did not approve. This was due to Matsunaga's abusive behavior towards Junko.

In 1985, Matsunaga convinced Junko that her family did not care about her after she had a suicide attempt. He persuaded her to move in with him. During this time, he purchased a building where he started a futon business. He was known for behaving erratically, such as claiming there was a spirit following people or using unusual religious references. By 1992, Matsunaga was involved in various fraudulent activities, stealing about 180 million yen, which is around 2.2 million US dollars, through blackmail and scams. At this time, he and Junko managed to evade police and were placed on Japan's most wanted list.

Futoshi Matsunaga lived in a condominium in Kokurakita-ku. His first victim was a married woman with three children. In April 1993, he persuaded her to leave her husband, claiming that Junko was his sister. One of her children died under unclear circumstances in September 1993. The surviving children went to live with their father and grandfather the following month. Matsunaga defrauded the woman of 11.8 million yen (about US$145,510). She died in March 1994, but the police could not prove Matsunaga's involvement in her death or that of her child.

Later in 1993, Matsunaga started preying on Kumio Toraya and his daughter. He blackmailed Kumio after Kumio shared details about his past criminal activities. Kumio and his daughter were held captive in Matsunaga's apartment. Matsunaga subjected Kumio to horrific torture, including electric shocks and forced humiliation. Kumio died from this treatment on February 26, 1996. Matsunaga tricked Kumio's daughter into believing she had killed her father. He ordered her and Junko to dispose of the body, which they threw into the sea near the Kunisaki Peninsula after destroying it.

Matsunaga targeted another woman connected to Kumio. He misled her by claiming to be a Kyoto University graduate and promised marriage. He swindled her out of 5.6 million yen (about US$69,066). This woman and her daughter were also trapped in his apartment. The mother escaped in March 1997 by jumping from the second floor. She was placed in a mental hospital, while her daughter was released.

In April 1997, Junko left for work but did not return home. Matsunaga contacted her family, issued threats, and blackmailed her mother using past abuse as leverage. He then faked a suicide to manipulate Junko into returning. He continued to abuse her and raped Junko's married sister, Rieko. The family eventually gave 63 million yen (about US$777,116) to Matsunaga, who then psychologically controlled them.

On December 21, 1997, Matsunaga forced Junko to administer electric shocks to her father, Takashige, until he succumbed. When Junko's mother, Shizumi, became mentally unstable from stress, Matsunaga ordered Rieko and her husband, Kazuya, to strangle her. A few weeks later, he commanded Kazuya to strangle Rieko while their daughter, Aya, assisted. Matsunaga and Junko confined Kazuya in a bathroom, where he died from starvation on April 13, 1998. Matsunaga then forced Junko and Aya to kill Rieko's 5-year-old son, Yuki. During the trial, Kumio's daughter testified against Matsunaga, stating she had also been tortured with electricity. Junko later claimed the girl's memories might be unreliable due to her guilt over their shared crime.

On June 7, 1998, Kumio's daughter strangled Aya. Matsunaga and Junko dismembered the bodies of their victims, boiling the remains in pots before disposing of them in restrooms or the sea. Matsunaga placed blame for the murders on Junko, who, with their two children, was the only surviving member of her family.

In July 2000, Matsunaga persuaded another woman to join him, promising marriage. By August 2001, she entrusted him and Junko with her twin toddlers. They soon convinced her to give them 20 million yen (about US$246,580) under the pretext of needing funds to care for her children.

Matsunaga's crimes came to an end when Kumio's daughter escaped on January 30, 2002. Despite being taken back into captivity on February 15, she managed to escape again on March 6 and reported the situation to the police. Authorities arrested Matsunaga and Junko the following day while attempting to reclaim the girl. The twins and the couple's children were placed in protective custody.

The media first reported that Matsunaga and Junko had held victims captive. However, more details about the murders emerged later. They were charged with several murders between 2002 and 2003. Despite Junko's admission to the killings, Matsunaga denied responsibility, claiming the women's testimonies were fabricated. As there were no physical remains or evidence, the investigation relied heavily on the daughters' testimonies.

On September 28, 2005, a court in Fukuoka sentenced Matsunaga and Junko to death. This included six murder cases, even though they were not directly responsible for Takashige's death. They appealed the decisions, but in September 2007, the Fukuoka High Court upheld Matsunaga's death sentence, changing Junko's sentence from death to life imprisonment due to Matsunaga's influence over her actions.