According to Daily Mail UK, the accused murderer was caught with his victim's liver in his trouser pocket and told police he had killed his elderly neighbour.
But the 24-year-old said he only killed Tan Loon-ubon, 79, after the old man claimed to have killed Mr Jongthong's father.
The murder took place at Mr Loon-ubon's home in the village of Dong Suang, Bueng Karn Province, in the northeast of the country.
Mr Jongthong told police he was visiting his neighbour as he often did, but on this occasion he had taken a tablet of methamphetamine and smoked some cannabis.
He says that their conversation took an odd twist when the elderly man suddenly said he had murdered Mr Jongthong's father.
Mr Jongthong said: 'He told me he killed my dad. I said that couldn't be true, but he wouldn't stop talking about it. I didn't know what to do.
'I stabbed him, and he cried and apologised. I was so angry. I asked him four times if he was telling the truth.'
Police say that the elderly victim was found with his abdomen cut wide open and had an injury to the neck.
A long kitchen knife found nearby is thought to be the murder weapon.
They said the victim's liver was found in Mr Jongthong's trousers. Asked whether he had intended to eat it, Mr Jongthong said: 'Yes, that's right.'
Thai media report that he even had a recipe planned for the liver, a spicy salad.
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