"There is nothing, nothing, that can prepare you for something like that," Mai Linh said. "There isn't a day that goes by when I don't think about how he died. The pain is unexplainable."

"The death penalty is a different kind of evil," she added. "One that will haunt me for the rest of my life."

The group now believes there to be more than 1,000 people on death row in the country -- among them foreigners from Laos, Cambodia, Singapore and Malaysia and a 73-year-old Australian woman convicted of drug trafficking.

The ruling Vietnamese Communist Party has long defended and justified its use of the death penalty. State documents released in 2017 showed approvals for lethal injections by the public security ministry.

CNN reached out to the Vietnamese government for comment on this story but received no response.

Top executioner in the region

Vietnam remains the biggest executioner in Southeast Asia, suggested by figures in Amnesty's report although the situation in Myanmar also got a mention.

"An alarming increase in the number of known death sentences was recorded in Myanmar, where the death penalty became a tool for the military in the ongoing and widespread persecution, intimidation and harassment of and violence on protesters and journalists," the report said.

Under martial law regulations, junta chief Min Aung Hlaing approves all executions and has the authority to overturn executions or commute death sentences to lesser penalties. There is "no appeal for decisions or convictions handed down" by a military tribunal, according to junta officials.

CNN reached out to junta officials for comment on this story but received no response.

"Data on the death penalty in Vietnam continues to be classified as a state secret," said Amnesty's death penalty expert Chiara Sangiorgio.

"[This] secrecy and the fact that media in the country are tightly overseen or controlled by the state has certainly contributed to a lack of information and international spotlight on the subject.

"We have written to the authorities seeking information, as we do for all countries that still retain the death penalty every year in preparation of our report. We did not receive any response."

The vast majority of Vietnamese death sentences are related to drug offenses, which according to Amnesty accounted for 93 out of the 119 death sentences handed down last year.

Others awaiting execution include once-powerful businessmen convicted of corruption, embezzlement and fraud. Nguyen Xuan Son, a former high-flying official who once served as the head of a major Vietnamese bank, was sentenced to death in 2017 for his role in a fraud involving millions of dollars in illegal loans. Others, too, have been convicted on state corruption charges.

Others on death row have been convicted of murder.

Last year, a court in the capital, Hanoi, upheld the death sentences of two brothers, Le Dinh Cong and Le Dinh Chuc, both farmers, for their roles in the killings of three police officers who were burned to death during violent clashes in their village.

Villagers blamed the violence on authorities' attempts to build a wall on farm land. Judges defended the death sentences, saying the brothers had ignored the law and "showed no respect" for the lives of security personnel.

Death penalty 'used to intimidate'

Vietnam's suspected rise to top executioner in Southeast Asia may come as a surprise to some but rights groups warn that executions in the country are likely to increase in the years to come.

"Vietnam continues to execute people at an astonishing rate," said Ben Swanton, advocacy director of the 88 Project, a non-profit organization advocating for free speech and human rights in the country.

"The Vietnamese Communist Party is aware that its use of the death penalty contradicts its narrative of the country being a peaceful and harmonious society and has the potential to damage its international reputation."

Human Rights Watch's Asia deputy director Phil Robertson called Vietnam "one of the worst rights-abusing states" in Southeast Asia.

"The death penalty in Vietnam is used to intimidate those who would break the law, while also showing the power of the ruling party," Robertson said. "This is a government that chases down dissidents, runs roughshod over civil society, sentences and imprisons people after kangaroo court trials, and now we know, executes far more people than anyone else in [the region]."

Comparing it to neighboring Singapore and Indonesia, countries that have made bigger headlines over the years with their execution cases, Robertson said: "Vietnam's horrendous record of executions dwarfs that of any of its neighbors but it is not surprising that the government has systematically implemented the death penalty and kept executions out of the public eye."