Nonthaburi Province, Thailand: Three iridescent coffins are mounted on the platform of a Buddhist temple on the outskirts of Bangkok. The two on the higher level hold the bodies of Somchit and Pornthip, a retired husband and wife murdered inside their home before dawn on Friday.

The coffin at their base holds the boy who did it – their grandson, Patchata Yimrueng, a 14-year-old responsible for what may be Thailand’s deadliest school shooting at the hands of a student.

Having killed the grandparents who had cared for him since his parents’ separation when he was two, Yimrueng locked the doors and went to Debsirin Nonthaburi School as usual, sitting in class on the sixth floor of building five without any obvious distress until period two was almost done.

Then, he pulled his grandfather’s 9mm handgun from his backpack. By the time he shot himself in building four, as police closed in, Yimrueng had killed or fatally wounded five teachers and a 12-year-old girl. Close to 30 were wounded, some critically.

The boy’s spectacled, ordinary-seeming face now half smiles from a photograph atop his coffin. Framed photographs of the grandparents he murdered sit on either side.

“We wanted to have the funeral together because the grandparents, they loved the boy a lot,” explains Somchai Chiagkongco, the boy’s other grandfather, on his mother’s side.

Somchai and his wife, Apinya, are in shock. They describe a good-natured grandchild with decent grades who was interested in basketball, swimming, running, badminton and, one day, becoming a banker like his late grandfather Somchit.

There is no obvious motive. Yimrueng, though quiet and maybe socially withdrawn, gave off no signals of being a mass murderer, they say.

“When we heard the news, it was about 12pm [on Friday],” Somchai says.

“I didn’t think for a moment that it could have been my grandchild. There are about 3000 students at the school. When I heard it was him, I could not believe it.”

Mother’s Day in Thailand is on Wednesday, and Somchai and Apinya say they had planned to pick up their grandson and take him into Bangkok city to see his mother. Instead, they are grappling with his death and the scale of his crimes.

“Last year, he asked to move in with us and change schools, but when we asked if he was having problems, he didn’t say anything,” Somchai says.

“If we knew he was having issues, we would have moved him. We would have been happy for him to live with us ... [but] he loved his grandfather [Somchit] a lot. This situation is unimaginable to us.”

Somchai and Apinya have no original theories about what motivated him to kill so many people, including those who loved him the most.

They have read in the media that maybe it was bullying, although no solid evidence of this has emerged.

Or maybe his late grandparents and teachers were riding him about homework and performance, as suggested by Thailand’s Prime Minister, Anutin Charnvirakul. It is all speculation.

“We want to say sorry to the community,” Apinya says.

“The community lost people. We lost a grandchild. We just didn’t see this coming.”