A killer was attacked by furious relatives for allegedly murdering his ex-girlfriend's father when she refused to reconcile with him.

Phanuphong Thararum, 28, reportedly took revenge on his former lover Anchalee Charoenying, 31, by stabbing her father dead in Chonburi, Thailand.

The suspect was caught on CCTV sneaking into a one-storey home where his former father-in-law Siwapong Boonprasert, 54, lived in the Bang Lamung District, early Thursday morning.

He entered the bedroom then plunged a six-inch blade into the older man's chest in front of his wife Rien, 53, and grandson. Rien screamed in terror as Phanuphong then fled the scene on his motorcycle.

Police arrived after neighbours reported the alleged murder at around 4:06 am. They found Siwapong's bloody corpse lying on a blue Doraemon-themed bed, with the knife still lodged in his chest.

Authorities tracked down Phanuphong through security footage, and arrested him at a rented room he shared with his mother around two hours later.

Footage shows the cops bundling Phanuphong into a pickup truck as Siwapong's furious family tried to attack him.

He was hauled to the Huai Yai Police Station, where he suddenly broke down in tears claiming he 'deeply regretted' what he did.

He allegedly admitted to police: 'I acted on a momentary impulse. I moved back to my mother's house less than a month after breaking up with Anchalee. I saw her posting about her new boyfriend, which made me angry and resentful.'

He added that he had been drunk during the attack and had initially targetted his ex-girlfriend. However, he turned his fury on her father instead upon finding that she wasn't home.

Police Captain Sitthisak Thornsawat, police station deputy inspector, said: 'We did not believe his testimony as he was still speaking incoherently and was heavily drunk.'

Phanuphong's mother, Sunan Thararum, 43, said her son woke her up that morning to confess to the suspected murder. She said: 'He had never shown any behaviour before that indicated he would do something like this.'

His ex-girlfriend Anchalee told police she ended the relationship because of Phanuphong's repeated drug use and violent behaviour.

Police said Phanuphong will be charged with murder as Chonburi forensic police collect evidence for further prosecution.