The judge overseeing the Emily Longley murder trial has told the jury a ''cool head is called for'' when they consider their verdict.
Justice Linda Dobbs began her summing up overnight (NZ time) and will continue today in Bournemouth, England.
The prosecution alleges Elliot Turner, 20, strangled the New Zealand teenager in his bedroom in the family home in Bournemouth on May 7 last year.
Turner denies murdering Longley, 17, but admits perverting the course of justice. His parents, Anita Turner, 51, and her jeweller husband of 22 years, Leigh Turner, 54, both deny perverting the course of justice.
Dobbs said the jury had to put aside any prejudice or sympathy when reviewing the case, the Bournemouth Echo reported.
She said it was up to the prosecution to prove that Turner was not acting in self defence and jury had to discern whether the force used by Turner was reasonable.
''Has the prosecution made you sure that Elliot Turner killed Emily Longley?''. If the answer was no, then Turner was not guilty.
She told them if they believed he had acted unlawfully then the question was whether he intended to kill her or cause serious bodily harm.
If they answer was yes, then he was guilty of murder. If no, then Turner was guilty of manslaughter.
Dobbs said the jury should treat Turner as a person of good character.
She also directed the jury on a number of points of evidence, including the covert conversations taped by police after Longley's death, the 999 call made by Turner's mother and claims the accused had practised chokeholds with a friend.
Earlier, while summing up the defence case for Turner, Anthony Donne, QC, said there was plenty of evidence that Longley was well capable of striking back.
He described Turner as "brash, flash, boastful, volatile, obsessive, possessive and obsessed".
The prosecutor, Timothy Mousley, QC, said Turner was a ''remorseless, controlling, possessive and vicious'' character who murdered Longley.