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Lead defence counsel Judith Ablett-Kerr QC said it was clear Weatherston (33) was responsible for killing Miss Elliott.
The issue for the jury was whether he was "a cold-blooded, premeditated killer" as the Crown alleged, or "a man who, as a result of provocation, had lost the power of self-control when he committed his terrible act, because it is a terrible act".
Crown counsel Marie Grills said there were no acts to cause provocation or loss of self-control.
The Crown case was that Weatherston not only intended to kill Sophie Elliott but also that he set out to mutilate features of her beauty and did so in a calm, collected and focused way.
Weatherston had taken a knife with him when he visited 22-year-old Miss Elliott's Ravensbourne home on January 9, 2008.
The knife was found broken and covered in blood after the attack.
Weatherston yesterday admitted committing manslaughter but not murder.
Mrs Ablett-Kerr said he was provoked by the emotional pain of his "torrid and tumultuous" relationship with Miss Elliott.
His unique personality meant he was ill-equipped to deal with it.
The defence would say Miss Elliott attacked Weatherston with a pair of scissors while he was in her bedroom, and he responded, losing self-control, Mrs Ablett-Kerr said.
The defence is arguing the former economics tutor was acting under provocation when he inflicted the more than 200 wounds.
Miss Elliott had been due to leave for a job with Treasury in Wellington the next day.
The jury of eight men and four women was told by Justice Judith Potter the partial defence of provocation was complex and, if accepted, it might reduce a verdict of guilty of murder to one of guilty of manslaughter.
The Crown would have to prove, beyond reasonable doubt, that Weatherston had acted without provocation, the judge said.
Weatherston's trial on the charge of murder began yesterday after a two-day delay caused by ongoing legal argument about disputed issues in the case.
When the charge was put to him, Weatherston replied he was "not guilty of murder, guilty of manslaughter".
Justice Potter warned jurors they were not to read or watch media reports about the trial.
The Crown will present evidence from 31 witnesses during the trial, which is expected to take about three weeks.
One of those called yesterday was Miss Elliott's mother, Lesley, who said the relationship between her daughter and the accused caused her some concerns.
Weatherston was taking Sophie for one of her economics papers, and she spoke to her daughter about the concerns she had about "professional boundaries".
She also told the court she felt the relationship was "strange" from the start.
It did not seem to be a loving one and was not "going anywhere".
Sophie had agreed with her about that, Mrs Elliott said.
Her daughter also talked to her about how Weatherston sometimes made her feel good and at other times made her feel like "shit".
He would tell her she was fat and ugly and never going anywhere, then would say she was beautiful and would "really go places".
"She found it confusing," Mrs Elliott told the court.
Her evidence continues today.