Robert Hollick, 43, from Upper Hutt, Wellington, was stabbed with a knife three times in his neck and back during a bar fight in Pattaya in August.
Police arrested a 40-year-old Swede, Andreas Ringvall, and charged him with murder. His Thai girlfriend was also detained.
Now, Mr Hollick's mother Anne Hollick has learnt Ringvall has been on the run for months, Stuff.co.nz has reported.
After the stabbing, Ringvall's girlfriend told police Mr Hollick started the fight, but witnesses said Ringvall caused the trouble after the woman began playing with Mr Hollick's dog.
The girlfriend was sent back to her room and Ringvall returned with a knife, stabbing Mr Hollick, they said.
Ringvall was bailed days after being arrested, but Ms Hollick said she was shocked to discover that he had been allowed to return to Sweden in late October for "family reasons".
The Foreign Affairs and Trade Ministry confirmed to her on Tuesday that Ringvall was missing.
Ms Hollick said she was told that after Ringvall's court date in November came and went, the New Zealand embassy sent the court an official letter demanding information but received no reply.
"I was a little angry that they should treat, not me, but New Zealand this way as though we were so insignificant they could do this without informing the New Zealand Government or whoever they're supposed to inform," she was quoted as saying.
She had never felt any anger towards Ringvall - "all that matters is that I don't have Robert any more".
"He's [Ringvall] got to live with what he's done so I can't waste time with all these mixed feelings . . . I'm more happy thinking of my son rather than him. He doesn't mean anything."
A ministry spokeswoman confirmed that the Pattaya Court had granted Ringvall permission to leave Thailand.
He had failed to return and an arrest warrant had been issued by the Thai authorities, she said.
The ministry would keep in touch with Mr Hollick's family and provide them with updates, while the New Zealand embassy in Bangkok would continue to seek information from the court.
The matter rested with the Thai authorities and the New Zealand Government was unable to intervene in judicial matters, she said.