On Sunday night, cyber-security expert and prolific blogger Kenichiro Okamoto held a seminar in the city of Fukuoka about writing on the internet. During the two-hour event, Okamoto — who also used the online handle Hagex — offered tips on how to get traffic, as well as how to deal with the “blog trouble” he had experienced online over the years, from readers pointing out typos to legal threats and internet feuds.

After the seminar, Okamoto went to the use the bathroom and was stabbed to death by an assailant, who then fled. According to The Asahi Shimbun, police say that a 42-year-old man named Hidemitsu Matsumoto later turned himself in, saying he “held a grudge” against Okamoto because of their online exchanges and had decided to kill him.

Prior to the arrest, an anonymous user with the handle Teino Sensei (or Mr. Idiot) posted that they planned to surrender and take responsibility for the crime. Teino Sensei had often clashed with Okamoto online; one post by Okamoto lists hundreds of aliases that Teino Sensei had used to harass him since 2016. In a post on May 2nd, Okamoto described how his blogging service, Hatena, had frozen Teino Sensei’s accounts whenever they were reported, though this did not stop the user from making new accounts. Okamoto continued that while the online harassment was “not a problem for people like me who are accustomed to abusive language, most people are frightened by it.”

Although police have not yet confirmed that Matsumoto is Teino Sensei, the murder is a disturbing reminder that online harassment has profound real-life consequences. Death threats aren’t any less disturbing just because they arrive on Twitter or in comment sections, and online conflicts have spiraled out to real-life violence before. Last December, an argument over a Call Of Duty match lead to the “swatting” and death of a 28-year-old man, who was shot to death by police after a fraudulent report that there was an ongoing hostage crisis at his home.