A New Zealand-born Sydney financier accused of torturing to death 17 rabbits and a guinea pig believed he was on a divine mission to save the animals and didn't realise his actions were wrong, a court has been told.
Brendan Francis McMahon, 37, has pleaded not guilty to 18 aggravated animal cruelty charges.
McMahon is arguing he should be found not guilty by reason of insanity because he was of "diseased mind" at the time of the alleged offences.
The dead or severely injured animals were found in or around McMahon's central Sydney office in July and August last year.
Treating psychiatrist Stephen Allnutt told the Downing Centre Local Court that McMahon had been "floridly psychotic" because of an eight-month addiction to the drug ice, or methamphetamine.
This psychosis, which McMahon was predisposed to through a family history of bipolar disorder, persisted after he stopped taking the drug, Allnutt said.
Six weeks after his arrest, Allnutt said McMahon observed, "I wonder if I made a mistake, because I never actually asked the rabbits if I could kill them."
"I believe he was doing it in a deluded mental state in the belief that what he was doing was right," Dr Allnutt told the court.
"He felt justified in doing something that he saw as morally right because he was delusional."
"That was the degree of distortion of his capacity to reason about rightful and wrongful, significantly compromised by his delusional belief that he was placed on the earth for a special purpose and that purpose was to free the animals."
McMahon believed he was a "tool for the universe", and in 2003 had become "quite preoccupied with mystical beliefs" that "birds were flying near to him and communicating", Allnutt said.
The court has previously heard, in two unsuccessful attempts to have the charges dropped on mental health grounds, that McMahon believed he could communicate with animals through a "third eye" and it brought him "a joy in his heart".
Allnutt said he was surprised McMahon's previous mental health applications had failed.
Crown prosecutor Laurie Gray asked the doctor if he believed McMahon could have been fabricating his mental illness.
"I spent a lot of time with him and my conclusion was that this man was not fabricating," Allnutt said.
He agreed that McMahon appeared to have held down employment during the period, but said that did not mean he was not suffering psychosis.
Defence barrister Douglas Marr told the court McMahon had been suffering from "a defect of reason due to a disease of the mind".
"As a result he didn't know that what he was doing was wrong," Marr said.
Gray argued McMahon's mental state was self-induced and the defence had failed to prove he was suffering from an underlying condition.
Magistrate Ian Barnett reserved his decision to July 3, and continued McMahon's bail.
McMahon has been ordered not to approach any pet stores or rural properties as a condition of his bail.