The gunman who killed the director of a hospital on Saturday told police that he shot the doctor out of anger after thinking he was overcharged for his visit while under the influence of drugs.

Police told the press that Denphum "Phum" Sangthong, 22, had confessed to the police that he had a personal grudge against the victim, Chumphon Decha-amphai, 58.

According to police, the suspect said he was a regular patient at Phra Chom Klao Hospital in Phetchaburi, where the victim was director, and that he had often complained about the hospital's poor services.

Mr Denphum said he became sick last week and went to the hospital director's private clinic instead, police said.

When he found out that he was charged 130 baht for a consultation, not 30 baht like at a public hospital, the suspect got angry and fired seven shots at the doctor, he added.

The suspect had admitted to spending 1,000 baht on amphetamines prior to his visit, and that he was under the influence of drugs when the shooting took place.

Police ruled out reports which said the gunman was angry because the doctor intervened in his romance with a girl who had worked at the clinic.