In 1980-1990s, a religious group committed an indiscriminate mass murder using Sarin gas on the subway in Tokyo, assassinated their protestors using VX gas. In addition, they tried to sprayed anthrax to their complainers in the neighborhood. Over 3000 followers were arrested. After over a decade of long trial, the leader and 12 followers were sentenced to death in 2011.
One day, the horrifying memory faded away from people in this high-tech world, suddenly, the 13 prisoners sentenced to death were executed unannounced all at once. It was July 2018.
The religious group was based on Buddhism and Hinduism. The leader just started a Yoga club in Tokyo in 1984. They gradually armed themselves for Armageddon and waged an all-out war against the government. It was during the high economy growth in Japan.