I'd actually urge you to look into the details of the case — Nagaenthran had two psychiatrists and a psychologist evaluate him on separate occasions, each of them certifying that he was of borderline intelligence, but not mentally ill (or 'mentally retarded', per DSM-5). The third examination established that Nagaenthran knew he was carrying drugs, that he was the member of a gang, that he was carrying out such an act because of 'a sense of loyalty and gratitude to his boss', that he had not been coerced into delivering the drugs, and that he was aware of the sentence for drug trafficking in Singapore.

Nagaenthran's lawyer disagreed with these assessments, but, unable to find a psych willing to certify Nagaenthran's intellectual disability, got so desperate that he signed an affidavit declaring himself convinced that Nagaenthran was intellectually disabled. The court rejected this affidavit after establishing that Nagaenthran's lawyer had no medical expertise and after he admitted that he was essentially 'speculating on what the appellant's mental age was'. Then the AG's office got a fourth medical assessment in 2021, which was blocked from submission to the court by Nagaenthran's lawyer, arguing that the 'appellant was interested in 'medical confidentiality'. At that point the court sort of threw up its hands and went 'you claim he's intellectually disabled but now you want to block medical reports on that exact issue, due to privacy reasons?'

Keep in mind that at this point, the case had been in Singapore's courts since 2010, and the various assessments and appeals had lasted more than a decade.

Nagaenthran's family, with the help of activists, requested appeals from both Malaysian's prime minister and a direct appeal to Singapore's president, who had the power to stay the execution by issuing a pardon. Singapore's president said she looked into it and was satisfied that the case was sound.

Finally, Nagaethran's family — again with the help of activists — filed a final appeal, arguing that the sentence was unconstitutional because Chief Justice Sundaresh Menon, who presided over Nagaenthran's previous failed appeals, was also the Attorney-General during his conviction.

At this point the panel of three judges threw the application out on the grounds that it was a 'calculated attempt' to diminish the finality of the court process. "No court in the world would allow an applicant to prolong matters ad infinitum" by filing such applications, said the judges. They'd been pushing such applications over the course of a full decade by that point, switching arguments three times.

Source: https://www.channelnewsasia.com/singapore/nagaenthran-dharma...

And read the full judgment: https://elitigation.sg/gd/s/2022_SGCA_26

I'm sympathetic to the goals of the activists — they want to push a test case to repeal the death penalty. The problem is that it doesn't seem to be a particularly good test case. Nagaenthran had successfully stayed his execution multiple times over the past decade, and he had pretty decent legal representation throughout. (Notwithstanding the affidavit — which probably harmed his case more than it helped).

You may argue that Singapore is a 'rather extreme outlier among developed countries' — but its policies on drug enforcement is 100% consistent with just about every other country in its vicinity. In fact, had Nagaenthran been caught in his home country of Malaysia, he would be sentenced to death just the same, since Malaysia has pretty much the same laws on drug trafficking. The activists have been raising international awareness of the case, but Malaysia's government is pretty blase about the whole thing, since they have similar laws and a similar justice system, and Malaysian anti-narcotics officers have a long history of working with Singaporean anti-narcotics officers to stop the drug trade from trickling into their countries from the Golden Triangle to their north.

Edit to add: I was actually prepared to get angry over this case, but then I read the full judgment and now I think ... "hmm, this is a more complicated case than I thought."