In a tape-recording played to court, William O'Brien, 58, claimed the final row came after a series of confrontations when Mr Scott became abusive.
Mr O'Brien, who lived in a bungalow at Mr Scott's stud farm in Cheveley, Cambridgeshire, has pleaded not guilty to murdering Mr Scott on 30 September last year.
He has admitted shooting his employer, but claims provocation. He has also denied threatening to kill the only witness to the shooting, Christopher Forster, a gardener. In the tape-recorded police interview the day after the killing, Mr O'Brien told of the meeting that ended with his employer lying dead in a barn after Mr Scott had told him he was "temporarily dismissed" and insisted he remove all his possessions from stud buildings, including the barn.
As he removed items from the barn, Mr Scott had queried whether any of them belonged to him rather than O'Brien. "I had my single-barrelled gun in there," O'Brien allegedly told police. "He was going on so much and shouting and that, that I just picked that gun up and fired it. I am sorry, but that's it."
Mr O'Brien wearing a tie and light grey pullover, sat looking at notes as the sound of his voice came from a large black box used for police tapes. "I don't know if it's in my interests or not, but Mr Scott has been away for treatment on three occasions that I know of," he said in the recording. "He was away for two months for mental treatment, one or two people told me he was schizophrenic."
O'Brien went on: "He was shouting and bawling. You could see the froth at the side of his mouth. He was sticking his finger in my chest." There was another shotgun in the room of the barn where the final row took place, but that was broken, he said. The usable gun was a 12-bore single-barrelled shotgun. "I picked the second gun up when he started to shout and turned his back, and I just fired it.
"I don't know what happened, I just went. I have had so much with that man since 1992, it's unbelievable.
"He has driven us around the bend with his language, his abuse, his shouting. Every time there is anything wrong he always comes digging at you. The man just goes berserk. Every other week there's something to fight about."
Mr O'Brien said on the tape played to Norwich Crown Court that he had worked with the previous owner of the stud from 1977: "I never had any trouble with that man."
He described how his workload had grown since Mr Scott took over in 1992, with days often beginning at 5.30am and continuing into the evening. "I didn't decide to shoot him," he declared. "I just took the gun up and fired it at him. He had finished me. He told me I was suspended, whatever that meant ... from that time yesterday evening."
He denied seeing Mr Forster standing nearby after the shooting and telling him: "The next one's for you."
O'Brien said that the final confrontation had its origins two days earlier at a bloodstock sale in Newmarket when Mr Scott became "very abusive" after hearing he had refused to make an appointment for potential buyers to view horses. Further excerpts from the interview tapes were later played to the jury.
The trial was adjourned until today.Reuse content