Shiraishi called himself "@hangingpro" on one of his five Twitter accounts and offered to help people in pain. He tweeted statements such as "It is not hard to hang oneself" and "If you cannot help yourself, I can help you," public prosecutors revealed during the trials.

But once he had lured the women to his apartment, he strangled them, sexually assaulted and robbed some of them, and cut up their corpses.

The news prompted Twitter to introduce rules against promoting or encouraging suicide and social harm on its service, and led Japan's government to expand telephone and online support for people contemplating suicide.

Shiraishi told police he had tweeted about suicide and exchanged messages with several women, coaxing them to his apartment, Japanese media reported.

Although his defence lawyers argued that he assisted the victims' suicidal wishes, Shiraishi later said he killed them without their consent.

In the ruling, presiding Judge Naokuni Yano said none of the victims agreed to be killed and that Shiraishi was fully responsible for their deaths, according to media reports.

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He said the crime was extremely heinous and had caused fear and concern in a society where social media had become an indispensable part of everyday life, NHK public television reported.

Japan usually reserves the death penalty for those convicted of multiple murders. Executions are carried out by hanging. Its suicide rate ranks among the world’s highest. Following a recent decline, the number has climbed back this year as people were hit by the effects of the pandemic.

In an interview with NHK in 2017, Twitter CEO Jack Dorsey called the case "extremely sad," and said the social media company needed to make sure its site "is being used in positive and healthy ways."