The 11-member jury, which was sent out to deliberate at 1.03pm, was earlier told to put aside feelings of horror or disgust at Sophie Elliott's injuries.
Justice Judith Potter said in her summing up in the High Court in Christchurch that they must reach their conclusion free of prejudice or feelings about the accused or any other person involved in the case.
They had to put aside any concern about Weatherston and any feelings of horror and disgust about the nature of the injuries.
They also had to put aside feelings of sympathy for the families of the accused or the victim, or opinions about the lifestyles of people involved in the case.
"You are the judges," Justice Potter said.
She continued her summing up talking about the charge of murder.
She referred to a questionnaire she had given to the jury.
The first question they must consider was: was Sophie Elliott killed?
Then, did Weatherston intend to kill her or cause injury he knew was likely to cause death and was reckless? And if so, did he have murderous intent?
They had to be sure he had meant to cause the injury, knew it would cause death and then recklessly went and did it anyway if they were to find him guilty of murder.
On provocation, she said the Crown must exclude it, and prove beyond reasonable doubt the accused did not act under provocation when he killed Sophie Elliott.
The law said if the accused was acting under provocation what was otherwise murder would be manslaughter.
She posed three questions the jury must consider:Was there something said or done by Sophie Elliott?
Did that thing deprive the accused of the power of self-control and induce him to kill Sophie Elliott?
In the circumstances of the case was what was done or said sufficient to deprive a person with the characteristics of Weatherston of the power of self-control?
She suggested they must treat the accused as having unusual characteristics unless the Crown had proved he did not have them.
The onus was on the Crown to prove Clayton Weatherston was guilty.
Justice Potter said the jury must start from a point of assuming Weatherston's innocence and had to be satisfied beyond reasonable doubt.
Weatherston (33), a former economics tutor, admitted stabbing his former girlfriend Sophie Elliott (22) in Dunedin on January 9 last year but said he was provoked. He had denied murder but admitted manslaughter.
It was not enough to say Weatherston was probably, or very likely to be guilty of murder; they had to be sure he was, Justice Potter said.
On the other hand, it was invariably impossible in cases like this to prove it outright.
In this case Weatherston had chosen to give his evidence, but the onus was not on him to prove his innocence.
The fact he gave evidence did not alter the burden of proof which rested with the Crown, she said.
The jury must decide solely on the evidence put to them inside the courtroom and had to decide on their own assessment if they believed witnesses and must apply the jury's collective common sense and experience.
She advised them to approach each issue on the strength of what they remembered and their own notes.
The jury did not have to accept the evidence of experts, it was up to the jurors to ultimately decide what facts to accept and how to apply them.
The jurors were the only one with the whole picture before them.
"You alone are best placed to evaluate the evidence and decide the ultimate issues in the case."
Intentions were always a matter of inference, because you could not go inside someone's head and know what they were thinking. Intention was an essential element of most crimes, especially murder.
The Crown said the nature and number of Sophie Elliott's injuries was a signal of intention.
If the evidence supported two inferences, they must take the stronger or more obvious; if they were of similar weight they must not choose between them because that would be speculating.
Circumstantial evidence relied on reasoning by inferences. There was nothing "second-rate or dubious" about circumstantial evidence. Taken together it could result in proof beyond reasonable doubt.
The jury must give its own weight to hearsay evidence such as that from many of the witnesses, including Miss Elliott's mother, her friends, Weatherston, her diary and so forth and give consideration to the fact it could not be cross-examined.
"But remember Sophie Elliott is not the person on trial, evidence about her had been admitted because it was relevant to issues in this trial."
Earlier Justice Potter told the jury the two pivotal issues for the jury were whether the Crown had proved Weatherston had acted with murderous intent, and if so, had it been proved he was not acting under provocation.
She has said she will talk about matters that apply to all criminal cases, the provocation defence, and summarise the cases of the Crown and the defence.
