A locksmith pleaded guilty Tuesday at the Tokyo District Court to illegally confining and murdering a freelance journalist in September.
Keizo Sakurai, 43, said he had to kill Satoru Someya, 38, because he believed the writer spread harmful rumors about him.
Sakurai is accused of conspiring with former professional diver Yoshihiro Kumamoto, 32, and former company employee Ryoichi Fujii, 34, to commit the crime.
Kumamoto and Fujii also pleaded guilty at the opening session of their trial Tuesday.
"If I did not kill him, I would still have been a victim of false accusations by the dirty writer, and would have suffered social ostracism," Sakurai said. "I could not help but kill him."
According to the indictment filed by prosecutors, Sakurai decided to kill Someya because he believed the writer had criticized him in his book "Kabukicho Underground," published last July, and refused to apologize.
Someya had praised Sakurai in a book he published in 2002, calling him the "ideal locksmith." Sakurai, expecting Someya to mention him in future books, paid the writer a gift of 1.5 million yen. Someya's next book, however, referred to a locksmith in the Kabukicho district of Shinjuku Ward who was involved in criminal activities.
The prosecutors said Sakurai became convinced that Someya was referring to him, and feared his job would be endangered if such a rumor spread.
To force an apology out of the writer, Sakurai abducted Someya with the help of Kumamoto and Fujii on Sept. 6 and confined him in a Kabukicho apartment. When Someya refused to apologize, the trio beat him before taking him by boat to Shin Kiba, Koto Ward, where they stabbed him several times and dumped his body in Tokyo Bay.