The main public gallery in the High Court at Christchurch was packed as the 33-year-old former University of Otago research fellow and teacher in the economics department entered the witness box on the 11th day of his trial for the murder of Miss Elliott.
For more than two hours, Weatherston answered questions from defence counsel Judith Ablett-Kerr QC about his childhood and family life, and the anxieties and insecurities he began experiencing even before he went to school.
He talked of his fear of being teased when he had to get glasses at the age of 11 and of how he later sometimes became ill over having to sit university exams.
He also spoke of his academic and sporting successes, of instructing classes in aerobics at a local gym and his role as Shaq the Cat, the Otago rugby mascot.
Just before court ended for the day, Weatherston described the start of his relationship with a young woman in 2004 and of a conversation they had in Starbucks one day early in the relationship.
The young woman, whose name is suppressed, had pointed out another young man she had slept with and he asked her directly how big the man's penis was.
"I don't know why I asked. I think I was after an ego boost for me," he told Mrs Ablett-Kerr.
But the way the young woman answered his question meant he "did not get a boost", he said.
Weatherston admits killing 22-year-old Miss Elliott, his ex-girlfriend, on January 9 last year, but says it was manslaughter, not murder.
He arrived at the Elliott house about 12.15pm with a knife in his computer bag.
Within 15 minutes, Sophie Elliott was dead, stabbed or cut 216 times with the knife and a pair of scissors.
Opening the defence case yesterday, counsel Greg King said Weatherston would say he was provoked and lost control after he asked if he should be tested for an STD, knowing Miss Elliott had slept with someone else while on holiday.
"He will say events really came to a head when Sophie, no doubt at her wits' end, screamed and lunged at him with scissors, knocking off his glasses, leaving him exposed and vulnerable."
Two eminent psychiatrists would give evidence the accused had a complex psychological make-up, with features of anxiety disorder and personality features of narcissism, was prone to narcissistic rage when frustrated or humiliated, and that he could be diagnosed with a personality disorder, Mr King said.
He told the jurors Weatherston's very poor eyesight - his optometrist yesterday described him as myopic in both eyes and needing to wear glasses full-time - was directly relevant to the defence of provocation.
Without glasses, he was particularly vulnerable because he could see only 15cm in front of his face.
While the Crown alleged the accused was a cold-blooded and premeditated killer who went to Miss Elliott's home, armed and ready to kill, the defence contended that was "ludicrous", Mr King said.
Provocation robbed the accused of self control.
He killed Miss Elliott because he lost control as a result of things she had done in the volatile, up-and-down relationship the pair had been in during the previous few months.
"It was a relationship too difficult for him to deal with. He was not psychologically equipped, because of his make-up, to deal with the events," Mr King said.
Weatherston had not gone to the house to kill but to return some gifts Miss Elliott had given him.
What happened in the bedroom was "the straw that broke the camel's back".
"He was gone. He had lost his ability to control himself . . . his power of self control," Mr King said.
"And you know what he did and it's horrific, just horrific. But the defence say it was the actions the a man who had lost his power of self control."
A verdict of manslaughter in no way trivialised or minimised what had happened.
The defence of provocation was a concession to the frailties of human nature when even ordinary, decent people could lose the power of self control, he said.
Nobody was suggesting Weatherston should be allowed to walk away from the courtroom a free man.
Mr King reminded the jurors the accused had pleaded guilty to manslaughter when the murder charge was put to him on the first day of the trial.
There was no suggestion from the defence that Miss Elliott was in any way to blame for what happened to her.
The defence of provocation focused on the mental functioning of Weatherston.
It was a very old defence, the equivalent of the French "crime passionel".
