The fast-tracked trial of Wichai Somkhayoyai, 24, and Bualoi Posit, 23, ended last Friday, only a dozen days after the killing of the 21-year-old student on Koh Samui island.
Homicide trials in Thailand can drag on for many months, which is why the haste to close the case raised eyebrows. But the men's defence lawyer, Amarin Nuimai, insisted there has been no rush, and that his clients had admitted conspiracy to rape, pre-meditated murder and hiding Ms Horton's body.
Pressure to find the killers was immense and came directly from the Prime Minister, Thaksin Shinawatra. He has already called for those guilty of the crime to face the "hardest punishment", which would be lethal injection.
"They deserve a most severe penalty because they have done huge damage to the country," he said, referring to the lucrative tourist industry, which accounts for as much as 6 per cent of Thailand's gross domestic product.
However, the execution of the two men would mark a break with custom in Thailand, where those who admit murder normally avoid capital punishment and receive a life sentence.
Wichai and Bualoi turned themselves in to the police when officers posing as fish traders boarded their trawler and said DNA tests would be carried out on the crew.
Wearing shorts and heavy shackles, the two sat stony faced in the courthouse of Surat Thani, while witnesses, including police officers, a forensics expert, fishermen and British tourists, testified on Friday.
Christopher Burrows, from Silsoe, Beds, recounted spotting the eerie glow of Ms Horton's mobile phone on the shore and, as he picked it up, hearing the faint cries of a woman saying "I'm sinking, I'm sinking." They were Ms Horton's final words.
Locating the mobile phone led investigators to check the fishing boats moored offshore, the court was told, which brought police to the two men's trawler.
A police officer told Judge Chamnong Sutchaimai that the two crewmen admitted watching pornographic videos and drinking whisky on their boat, moored opposite Lamai Beach in Koh Samui, where Ms Horton was staying.
They said they had paddled ashore, using plastic petrol cans as floating devices, and after spotting Ms Horton alone on a moonless beach and chatting on her mobile telephone, began to stalk her.
They then crept up behind the student, struck her over the head with a wooden beach umbrella pole and dragged her away. After taking turns to rape her they then tossed her into the sea, where she drowned. She had suffered 21 wounds and two blows to the skull.
Afterwards, the men boasted to shipmates about having raped a foreign woman. The boat's mechanic said that when Bualoi returned to the vessel, he had shouted: "I'm drunk and today I have done delicious, very delicious things." Ms Horton's body was discovered by a jet-skier on 2 January, after drifting to a remote beach a few kilometres from Lamai.
The court heard that evidence from DNA tests, witnessed by British embassy officials, was compelling.
Pattana Kitkailart, the physician who administered the tests, said there was only a one in six billion chance that the accused were not guilty.
Bualoi told his lawyer: "If I could turn back time I would not have done it. Everything happened because of alcohol."
The brutality of the murder has appalled the Thai public, many of whom have followed the details of the case in the local press. "It was horrible that Katherine's own mother overheard her last screams on a mobile phone," Kaek Inparawut, a former receptionist at a Koh Samui guesthouse, said. "Ten years ago European girls would get drunk and sleep on the beach with no problem. Now there are lots of outsiders around and things have changed."
At least two other foreign tourists have been raped on Koh Samui since 1 October, and Thai police are investigating possible links to the fishermen. The victims were a British woman, aged 26, who filed a complaint last October, and a Swedish girl aged 12.
A makeshift shrine was erected on Lamai Beach last Thursday after a Buddhist blessing service and bouquets were laid near the spot where the psychology undergraduate was attacked. Ms Horton's funeral will take place at St Isan's Church, in Llanishen, Cardiff, on Tuesday, followed by a private cremation ceremony.