Yamaguchi was a member of a radical group. In 1960, he killed Inejiro Asanuma when he was at a public meeting. Soon after, Yamaguchi killed himself in a juvenile center. He wrote on the wall of the room in which he hung himself. His last words were written using a paste made from tooth powder and water. He wrote "Seven lives for my country ..." which was a reference to the last words of 14th century samurai Kusunoki Masashige.