Open this photo in gallery RobertSchellenberg appears in court for a retrial of his drug smuggling case in Dalian, Liaoning province, China, Jan. 14, 2019, in this handout photo, Jan. 15, 2019. HANDOUT/Reuters

The rapid completion of the retrial and sentencing of Robert Schellenberg was “unusual and rare,” his lawyer says, as criticism among advocates erupted inside China over the legal treatment of a Canadian man sentenced to death for drug trafficking amid a widening diplomatic rift between the two countries.

“This case is very unusual. Every single step was unusual,” Zhang Dongshuo said in an interview.

Chinese prosecutors and courts initially spent more than four years assessing Mr. Schellenberg’s guilt, eventually sentencing him to 15 years in prison in November, 2018. The delay came in part because lower courts thought the evidence against him was insufficient and sought guidance from China’s Supreme Court, according to Mo Shaoping, a prominent Chinese human-rights lawyer who responded to a Canadian embassy request to secure representation for Mr. Schellenberg.

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Then, in the span of less than two months, judicial authorities ordered a retrial, conducted on more serious charges and, little more than an hour after the trial concluded Monday, said Mr. Schellenberg should be executed instead.

The case has roused “the suspicion that the judiciary in China is merely a servant of politics, and it hurts the international perception of China being a country governed according to the law,” wrote Zhang Jianwei, a professor at the country’s prestigious Tsinghua University, in one searing analysis of the case posted to China’s Facebook-like WeChat. “As such, there is more to be lost than gained. You may think you are doing good for the country, but you are in fact ruining it.”

The debate has made Mr. Schellenberg, who has previously been jailed for possession for the purpose of drug trafficking in British Columbia, into an unlikely icon for judicial fairness in China.

“In this country, administrative officials can make wrong decisions and diplomats can blatantly lie,” wrote He Weifang, an outspoken law professor at Peking University, in a post to his WeChat account. “But if judicial organs also take part in such a farce, succumbing to external interference and treating the law like a toy, that’s really a despairing and perilous situation.”

It was “a death sentence handed down with mysterious haste,” wrote Ma Gangquan, a human-rights lawyer, in a WeChat post.

The comments from Prof. Zhang, Prof. He and Mr. Ma were translated and posted online by China Change, which compiled Chinese legal commentary on the case.

The rapid chain of events unfurled mostly after the arrest in Vancouver of Huawei executive Meng Wanzhou, and critics have accused China of rushing justice in hopes of using the threat of Mr. Schellenberg’s execution as a tool in its efforts to press Canada for Ms. Meng’s release.

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The Chinese government has said it followed the country’s law in punishing Mr. Schellenberg. “There were no procedural violations,” the state-run Legal Daily reported, after speaking with a representative of the Dalian Intermediate People’s Court.

Mr. Mo agreed. “His rights, such as meeting lawyers, meeting consular officials, receiving translation assistance – all of these were secured,” he said in an interview.

But the handling of the case was nonetheless “very abnormal. I am referring to the speed. How can it be so fast?” he said in an interview.

Mr. Mo has never before seen an instance in which a case was sent to retrial, only to emerge with a death sentence so shortly after the conclusion of the new trial. Normally, “Chinese legal organs are extremely careful in the use of the death penalty – this has been their practice for quite a long time. So having an announcement of this kind is extremely rare.”

Another oddity: Mr. Schellenberg was accused of masterminding a drug-trafficking conspiracy with several other people, one of them a man named Mai Qingxiang. Mr. Mai was found guilty of trafficking more than 500 kilograms of methamphetamine – more than double the quantity police found in a warehouse Mr. Schellenberg had visited. But Mr. Mai was sentenced to death with a two-year reprieve, allowing him to escape execution if his conduct is good; Mr. Schellenberg received no such reprieve.

“The apportionment of punishment doesn’t make much sense,” Mr. Mo said.

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Not so, argued Hua Chunying, China’s Foreign Ministry spokesperson, on Wednesday. “China’s sentencing is just,” she said.

She drew on the ugly history in China of the Opium Wars, and the social devastation wreaked by the narcotics brought in to China by the British. Those memories “remain fresh to the Chinese people. We won’t allow drug dealers from any other country to harm the lives of Chinese people,” she said.

Canada has requested clemency for Mr. Schellenberg, Foreign Minister Chrystia Freeland said, calling the death penalty “inhumane and inappropriate.”

Ms. Hua’s response: “If you think it’s not humane or appropriate to sentence Schellenberg to death for smuggling 222 kilograms of drugs, then do you think it’s appropriate and humane to let drugs deprive people of their lives and happiness?”

But Mr. Schellenberg “insists that he is innocent” and wants to file an appeal, which will likely be submitted next week, Mr. Zhang, his lawyer, said. (Mr. Zhang works for Mr. Mo’s law firm.)

Mr. Schellenberg showed little emotion when the Chinese court read his death sentence, likely in part because Mr. Zhang had warned him about the possibility. On Monday, the court discounted Mr. Zhang’s arguments and based its ruling instead on the precise specifics of the prosecution’s case.

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It’s not clear an appeal will find a more sympathetic hearing in a country with a conviction rate in excess of 99 per cent.

“Since the court did not take into account any of my defence arguments in the previous trial, I am not sure if they will consider them in the future, or whether I can make any difference in the outcome,” Mr. Zhang said.

But as Mr. Schellenberg’s lawyer, “all I want is to try and get him a better sentence.”

With reporting by Alexandra Li