Seiichi Endo was born on 5 June 1960 in Sapporo, Hokkaido, Japan. From an early age, Endo was recognized for his academic aptitude, particularly in the sciences. He pursued veterinary studies and later went on to attend the prestigious Kyoto University, where he studied virus and genetic engineering in graduate school. His academic background and specialization in biochemistry and microbiology made him a prime candidate for involvement in biological and chemical research.

Despite a promising academic career, Endo's life took a radical turn when he became involved with the doomsday cult Aum Shinrikyo, led by Shoko Asahara. The cult blended elements of Buddhism, apocalyptic prophecy, and pseudoscience, and attracted many educated individuals disillusioned with modern society. Endo’s scientific knowledge was seen as a valuable asset by Asahara, who envisioned the creation of weapons of mass destruction to facilitate the group’s belief in an impending apocalypse. Endo soon rose in the cult’s internal hierarchy, becoming the head of Aum Shinrikyo’s “chemical brigade,” responsible for the development and production of nerve agents and toxins for use in terrorist acts.

Seiichi Endo played a pivotal role in one of the most horrific terrorist attacks in modern Japanese history — the 1995 Tokyo subway sarin gas attack. Ordered by cult leader Shoko Asahara, Endo was tasked with producing the deadly nerve agent sarin, which was later released by Aum members into packed subway cars on the morning of 20 March 1995. The coordinated attack during the rush hour killed 13 people, severely injured 54, and caused temporary vision problems and exposure symptoms in over 5,800 others. It paralyzed Tokyo’s transit system and shocked the nation.

In addition to the Tokyo subway attack, Endo was believed to have been involved in the production of other chemical agents used in Aum Shinrikyo’s earlier crimes, including sarin attacks in Matsumoto in 1994 that killed eight people and injured over 200. His role as the cult's chief chemical engineer made him responsible for manufacturing sarin under laboratory conditions at the cult’s facilities. Investigations revealed that Endo used organophosphate synthesis methods that required advanced technical skill and knowledge, underscoring the deadly capacity Aum had developed internally.

Endo was arrested on 5 May 1995, several weeks after the Tokyo attack, as Japanese authorities conducted mass raids and arrests of Aum Shinrikyo leaders and members. He was charged with murder, attempted murder, and violations of the Chemical Weapons Convention Implementation Act. On 16 March 2004, the Tokyo District Court found him guilty on all counts and sentenced him to death by hanging. His appeals were rejected by higher courts, and he remained on death row for over a decade.

On 6 July 2018, Endo was executed alongside six other senior Aum Shinrikyo members, including cult leader Shoko Asahara. The mass execution marked the Japanese government’s final chapter in the prosecution of the cult’s core leadership. His death came more than 23 years after the sarin gas attack, symbolizing both the closure of a national trauma and the dangers posed by scientifically trained individuals working under extremist ideologies.