Masanori Sumida's early life remains largely obscured, with few verified details available to the public. What is known is that he became deeply involved with Miyoko Sumida, the notorious ringleader behind the Amagasaki serial murder case. Together, they operated a disturbing and highly manipulative scheme that spanned decades and crossed multiple prefectures in Japan. While Miyoko served as the dominant figure, Masanori played an integral role in maintaining the environment of fear, control, and abuse that defined their so-called "pseudo-family."

The pair built a life around exploiting vulnerable individuals, often by isolating them from their families, coercing them into servitude, and manipulating legal structures through forced adoptions and marriages. The people drawn into this circle often had no prior relationship with each other, but were slowly turned into pawns in a system of psychological manipulation. Masanori, though not the primary face of the operation, was reportedly present throughout much of its execution, carrying out acts of violence, intimidation, and coercion.

Many of the families involved were drawn into the Sumida orbit under false pretenses and slowly stripped of their autonomy and dignity. Victims were often robbed of their possessions, threatened, beaten, or forced to abuse each other under the threat of worse consequences. Masanori, while under Miyoko's influence, is believed to have knowingly participated in these cycles of abuse for over 20 years.

The Amagasaki Serial Murder Incident began with the disappearance of a woman around 1987 and evolved into a horrifying series of murders, torture, and abuse that spanned nearly 25 years. Masanori Sumida, alongside other accomplices, helped sustain this cycle of violence, which involved hijacking multiple families and subjecting them to unspeakable cruelty. These families were psychologically broken down, separated, starved, beaten, and even forced to commit violence against each other under threats or manipulation.

The crimes remained hidden until November 2011, when a woman escaped from captivity and reported her abuse to police. This sparked the arrests of Masanori and Miyoko Sumida. Investigators soon uncovered a disturbing pattern of abuse and murder that spread across seven Japanese prefectures, including Hyogo, Kochi, Kagawa, Okayama, Shiga, and Kyoto. In total, eight deaths were confirmed, with more victims still missing. Victims included elderly relatives, young adults, and even children—many of whom were initially adopted or lured into the pseudo-family.

Evidence later revealed that the group’s strategy involved forcing victims into debt, isolating them from loved ones, and gradually taking over their lives, assets, and identities. Many were coerced into signing over financial resources, and several died through starvation, untreated injuries, or direct violence. Masanori’s involvement led to his arrest and indictment, alongside other members, for several key murders and body disposals.

Following Miyoko Sumida’s suicide on December 12, 2012, investigators struggled to fully untangle the conspiracy, but continued their efforts. Masanori was among those sentenced to life in prison or lengthy terms due to his role in the murders of individuals such as A’s second son, D’s eldest daughter, and the confinement deaths of others like G and F.