Mutsuo Toi was born on March 5, 1917, in Okayama Prefecture, Japan. He came from a well-off family. Tragically, his parents both died of tuberculosis when he was just a baby. After their deaths, Mutsuo and his sister were raised by their grandmother.

As a child, Mutsuo was outgoing and sociable. However, when he turned 17, he became more withdrawn. This change came after his sister married in 1934. Mutsuo continued to live with his grandmother.

He developed an interest in various stories, including that of Sada Abe, a woman known for a shocking crime that made headlines in Japan. Around this time, he began writing a novel titled "Yūtokaiōmaru". Mutsuo was also involved in a rural tradition called "Yobai," which involved sneaking into young women's homes at night.

His life took a serious turn when he was diagnosed with tuberculosis in May 1937. This illness impacted his social life significantly. He felt rejected by the young women in his village, as they distanced themselves from him after learning about his condition. Mutsuo faced feelings of isolation and despair. He expressed these sentiments in several suicide notes, revealing the pain he felt from being shunned by his community.

On the night of May 20, 1938, Mutsuo Toi severed the electricity line in the village of Kamo, plunging the area into darkness. Around 1:30 a.m. on May 21, he attacked his 76-year-old grandmother while she slept, decapitating her with an axe. Following this, he equipped himself with a modified shotgun, a katana, an axe, several daggers, and 200 rounds of ammunition. He strapped two flashlights to his head and moved stealthily through the village.

During the next hour and a half, Toi killed 29 of his neighbors. Twenty-seven people died at the scene, while two others later succumbed to their injuries. This brutal attack nearly wiped out half the small community. By dawn, Toi took his own life by shooting himself in the chest on a nearby mountain. His actions resulted in a total of 31 deaths, including his own and that of his grandmother.

After the massacre, many families left the village. The community suffered a significant decline in population, with reports stating that no one moved back to the area since the massacre. The house where Toi lived was left abandoned for many years and was eventually demolished in 2015.