Kumatarō Kido was born around 1857 in Japan during the late Edo to early Meiji period, a time of significant social and economic transition. Historical records indicate that Kido lived as a working adult with modest means and entered into a common‑law marriage, a form of partnership not uncommon at the time. The relationship deteriorated when his partner left him for Torajirō Matsunaga, a rival whom Kido believed had wronged him both personally and financially.

Kido’s grievances deepened after Torajirō’s brother, Denjirō Matsunaga, allegedly assaulted Kido and defrauded him of money. These events created a combination of emotional distress and financial loss that became central to Kido’s later actions. Rather than seeking legal redress, Kido resolved to pursue violent retaliation. He enlisted Yagorō Tani, a younger associate described in sources as his pupil, to assist him in a planned act of revenge against the Matsunaga family.

On 25 May 1893, Kumatarō Kido and Yagorō Tani carried out a coordinated killing spree in what became known as Kawachi Jūningiri (河内十人斬り), translated as “the Kawachi ten‑person slaying.” Armed with Japanese swords and hunting guns, the pair began their attacks by targeting the household of Denjirō Matsunaga, where they killed four people. The assault was deliberate and swift, indicating prior planning and familiarity with the victims’ residences.

After the initial killings, Kido and Tani proceeded to the home of Denjirō’s son. There, they murdered five additional people, continuing the spree without interruption. The victims included an infant, underscoring the indiscriminate nature of the violence once the attacks began. The killers also sought out and murdered Kido’s former common‑law wife and her mother, bringing the total number of victims to eleven.

Despite Kido’s stated intent to eliminate the entire Matsunaga family, the primary rival Torajirō Matsunaga was not killed, either because he was absent or managed to evade the attackers. After completing the spree, Kido and Tani took their own lives, bringing the violence to an end. Their bodies were not immediately found; instead, their remains were discovered on 7 June 1893, nearly two weeks after the murders.

The Kawachi Jūningiri shocked the local population and entered Japanese criminal history as one of the most notorious spree killings of the Meiji era. Because both perpetrators died by suicide, there were no trials or formal judicial findings. Contemporary reporting and later historical accounts framed the 사건 as driven by a mixture of personal betrayal and financial grievance, rather than political or ideological motives.

The case continued to resonate in Japanese cultural memory. It later inspired literary reinterpretations, most notably the novel Kokuhaku, which drew on the events and their psychological underpinnings. While artistic works expanded on themes of obsession and revenge, historical sources consistently emphasize the factual sequence: a targeted vendetta that escalated into mass killing.