A man felt like he was "in a movie" when he used a chainsaw to kill a former councillor, a court has heard.
Jonathan Stasiuk, 60, said he had no control over his actions as he attacked Gerry White, 73, at Lake Community Gardens on the Isle of Wight in May.
Southampton Crown Court heard a diagnosis of autism has helped him understand his actions.
Mr Stasiuk, of Sandown, denies murder but admits manslaughter on the grounds of diminished responsibility.
The court heard one psychiatrist diagnosed the defendant with autism following the attack and another suggested he has only "autistic traits".
Mr Stasiuk, a chef, and Mr White, publican and former Conservative councillor, were both trustees of the gardens.
Jurors were told Mr Stasiuk believed Mr White acted dishonestly over a £3,000 donation, had bullied his partner and used the gardens as a "dumping ground" for items from his business.
Giving evidence, Mr Stasiuk said he was having a "very relaxing day" on 27 May and had not been thinking about the dispute when he spotted Mr White.
"I honestly thought this was my time to get bullied," he told the court.
He said he took a chainsaw from the clubhouse and followed Mr White down a pathway to the lake.
"It was like a movie," he told the court.
"I didn't have any control over it. I was almost watching myself as being in the situation."
Mr Stasiuk then described how he cut Mr White with the chainsaw before strangling him.
The defendant said hearing expert evidence about autism during the trial had helped him to "understand the decisions" he made that day.
Asked what he made of the diagnosis, Mr Stasiuk said: "I found it very hard to believe.
"I have been to university, I had quite a high intelligence - it just never occurred to me."
The trial continues.