William O'Brien, 58, "perhaps did not have the mental powers and resources" to cope with the news that his career in racehorse breeding was over, said Christopher Leigh QC.
He was summing up on the sixth day of the trial at Norwich Crown Court at which Mr O'Brien has pleaded not guilty to murdering his employer on 30 September last year.
Mr O'Brien, who lived at Glebe Stud, Cheveley, Newmarket, Cambridgeshire, has also pleaded not guilty to threatening to kill the gardener there, Christopher Forster.
His defence has accepted that Mr O'Brien is guilty of manslaughter, having fired a 12-bore shotgun into Mr Scott's back after an argument. They claimed he was not guilty of murder because he was provoked by Mr Scott calling him an "Irish bastard". Mr Leigh told the jury that Mr O'Brien had little life outside the stud. The jury has to decide whether he was "a cold-blooded calculating killer or an ordinary bloke who finds his world falling apart".
James Hunt QC, summing up for the prosecution, said: "This case is a tragedy ... it is no manslaughter. It is murder."
Mr Justice Blofeld said the jury would be sent out today to consider its verdict.Reuse content