A Thai woman alleged to be among the worst serial killers in the kingdom’s history has been convicted and sentenced to death for poisoning a friend with cyanide, in the first of her 14 murder trials.

Sararat Rangsiwuthaporn, 36, described as an online gambling addict, is accused of swindling thousands of dollars from her victims before killing them with the chemical.

A court in Bangkok convicted her on Wednesday of fatally poisoning her friend Siriporn Khanwong. The two met up near Bangkok in April last year to release fish into the Mae Klong river as part of a Buddhist ritual. Siriporn collapsed and died shortly afterwards and investigators found traces of cyanide in her body.

Police were then able to link Sararat to previously unsolved cyanide poisonings going back as far as 2015, officers said.

“The court’s decision is just,” Siriporn’s mother, Tongpin Kiatchanasiri, told reporters after the verdict. “I want to tell my daughter that I miss her deeply and justice has been done for her today.”

Police said Sararat funded her gambling addiction by borrowing money from her victims – in one case as much as 300,000 baht (nearly £6,800) – before killing them and stealing their jewellery and mobile phones.

After she was charged, the deputy national police chief, Surachate Hakparn, said: “She asked people she knows for money because she has a lot of credit card debt … and if they asked her for their money back, she started killing them.”

She lured 15 people, one of whom survived, to take poisoned “herb capsules”, police said. Sararat faces 13 more separate murder trials and has been charged with about 80 offences in total.

Sararat’s ex-husband, a police lieutenant-colonel, was sentenced to 16 months in prison, and her former lawyer to two years for complicity in Siriporn’s killing, the lawyer for the victim’s family said.

Thailand has been the scene of several high-profile criminal poisonings.

Earlier this year, six foreigners were found dead in a luxury Bangkok hotel after a cyanide poisoning believed to be connected to debts worth millions of baht.