Mutsuo Toi was celibate and preferred not to be, due to tuberculosis. He took part in "Yobai" or "night-crawling", a rural custom which involved creeping into young women's bedrooms during the night to seek sexual intercourse. Apparently after contracting tuberculosis, the local women spurned his advances, thus making him incel. He was also bullied and suffered mistreatment from his neighbors after his diagnosis.
He proceeded to tragically murder 30 and injure 3 in May 1938 after cutting the electricity supply to his village, then going house to house with his headlamp previously used for "Yobai", killing residents with a sword, axe and a semi-automatic Browning hunting shotgun, seeking revenge on his community for what he saw as their mistreatment of him. This attack was known as the Tsuyama Massacre and was one of the worst rampage killings in the 20th century, it being the largest mass killing committed by a single individual in Asia until the 1982 massacre carried out by the South Korean police officer Woo-Bum Kon; half of the residents of Toi's small village were killed in the rampage.