His lawyer Marie Dyhrberg asked him what he was thinking.
Fleming responded: "I wasn't thinking - that was the problem."
But police said Fleming wanted Tasman dead and the cat was still alive when he put it in the plastic bag.
Constable Matthew Bunce told the court that soon after the incident Fleming said he "chucked it under some cars to make sure it was dead. It was buggered ...".
Mr Bunce asked Fleming why he didn't take Tasman to the vet to be put down humanely. Fleming said throwing it under cars was the "quickest way" and he had never thought of taking the animal to the vet.
Fleming said that at one point the cat rolled out of the bag while local children watched on.
"I realise what it looked like to the kids," he told the court.
He said he later apologised to a "little girl" for what he had done.
Fleming later put the cat in a rubbish bin outside a dairy.
He told police he put it there "because that's where I put all my rubbish".
Judge Heemi Taumaunu rejected Fleming's evidence.
He said a vet who examined Tasman's body concluded that the cat had multiple fractures and had been dead only about an hour.
Judge Taumaunu said Fleming's own evidence showed that he was trying to kill the cat when he flung it under cars.
"To put it simply, Mr Fleming's behaviour was bizarre."
Judge Taumaunu granted Fleming bail until he is sentenced in March but imposed a condition that he cannot own a domestic animal.
He faces a maximum sentence of five years in jail and a $100,000 fine.
Earlier, Ms Dyhrberg opposed a media application for cameras in court. She said her client had been attacked outside his home shortly after his arrest.
She said Fleming was caught from behind, called a "cruel bastard" knocked out.
Ms Dyhrberg said there was a risk of a vigilante action if her client's photo was published.
She also said he hoped to return to his profession as a counsellor and social worker.
But Judge Heemi Taumaunu said those who allegedly attacked Fleming already knew what he looked like and where he lived.
At the end of the day, the principles of open justice outweighed those concerns, he said.
Speaking about the case, SPCA executive director Bob Kerridge said his organisation felt it was about time the judiciary looked at handing down fines and prison terms to people who abused animals.
"To our minds, it is not going to stop until the judiciary start to get heavy handed on it."
Mr Kerridge said anyone who found an injured animal should take it to the SPCA where it would be treated free of charge.