His father remembers Alexandre Cazes as a gifted, trouble-free young man while the U.S. Department of Justice considered him a cybercriminal.

What led the 25-year-old Quebecker into being indicted as the mastermind of AlphaBay, the world's biggest online marketplace of illegal drugs, guns and stolen credit-card data, might never be fully known since he died shortly after his arrest.

However, some insight into the mind of Mr. Cazes can be found on an online forum where he portrayed himself as having permanently moved away from what he considered a dysfunctional society.

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He settled in Bangkok, learned to speak Thai, became wealthy and married a local woman. At the same time, he still considered himself a player, boasting how he cheated on his wife, using elaborate subterfuges.

The details come from posts made on the Roosh V forum, an online discussion board hosted by self-styled pickup artist Daryush Valizadeh.

Using the moniker Rawmeo, Mr. Cazes posted frequently on the forum, where members debated seduction techniques, compared women of various ethnicities and complained about feminism.

At one point, some forum users wouldn't believe Rawmeo's claim that he drove a Porsche and challenged him to explain how he became so rich so quickly.

The source of Mr. Cazes's wealth only became apparent when he was arrested earlier this month, on allegations he had amassed $23-million (U.S.) in net worth from running AlphaBay. A week later, he was found dead in his cell on July 12, an apparent suicide, just before a hearing to extradite him to the United States.

The next day on the forum, the administrator, whose handle Roosh suggests that it was Mr. Valizadeh, posted an update. "I have recently learned about the unfortunate passing of Rawmeo … May he rest in peace."

Rawmeo's most discussed forum contribution was a 7,500-word post last year about dating and "wife-hunting" in Thailand.

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He said that, unlike most foreigners who settle for promiscuous women in tourist areas, he had lived in Thailand since 2012, looking for "marriage-material" women, preferably virgin and from high society. He had attended a language school to become fluent in Thai, opened bank accounts and purchased houses, and drove a Porsche Panamera because "Thai girls love supercars."

Other members were skeptical. "Your constant preaching about virgins and high-class Thais is starting to get old," one wrote.

Mr. Cazes then sent them personal messages, with photos and video of his car and a pile of cash.

While Rawmeo's real name doesn't appear in the forum, the incident is mentioned in an asset-seizure complaint filed by U.S. authorities against Mr. Cazes.

"Cazes boasted to fellow RooshV users about recently buying a Porsche Panamera and, in response, fellow users challenged Cazes' claims of wealth and ownership. In an effort to authenticate his Porsche ownership, Cazes ... sent a video of himself driving the Porsche," the court document said.

On that discussion thread, another member, who called himself Lavidaloca, said that Rawmeo's claims that he became rich by investing in bitcoin currency didn't add up. Lavidaloca calculated how much Rawmeo would have earned, based on what he disclosed in previous posts.

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"The discrepancy in return is massive," Lavidaloca wrote.

Mr. Cazes replied that he hadn't disclosed all his sources of income. "We all have business, we all have our own ways to get some money."

In other posts, Mr. Cazes described his native Quebec as a place where too many are on welfare and where Muslim refugees get in easily and "start breeding like bedbugs" while he had trouble getting a visa for his Thai relatives.

"I left a broken society to live in a traditional one," he said.

He said he once dated a Vietnamese woman who had lived in the U.S. and she didn't think she was only good for cleaning and cooking. "Such a beautiful girl, who got poisoned by feminism," he said.

While he was married and expecting a child, he wrote that "I am what we call a professional cheater. Despite having a live-in wife, I have a secondary residence that no one knows about – and that's where I bring girls."

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He detailed ways to hide his cheating, such as having separate Internet phone apps, separate SIM cards and a prepaid anonymous credit card.