She added: "A cool head and a dispassionate appraisal of the evidence is what is called for."
The jury must now decide whether they are sure that Elliot Turner killed Emily Longley and whether he did so unlawfully.
Justice Dobbs said: "If you are not sure he was acting unlawfully, that is the end of the matter and you find Elliot Turner not guilty."
But if the jury are sure, they also have to decide whether Turner intended to kill Emily or cause her serious bodily harm.
The judge added: "If the answer is yes, he is guilty of murder. If the answer is no, he is not guilty of murder, he is guilty of manslaughter, which is an alternative verdict."
Earlier, the court heard how Turner's mother was faced with "the horror of a lost life".
Anita Turner, 51, is charged with intending to pervert the course of justice by removing and destroying evidence connected to Emily's death.
In Winchester Crown Court yesterday, Anita Turner's lawyer, Fern Russell, rejected allegations her client had "put on an act" when she called emergency services on the morning she discovered Emily dead.
Making her closing speech, she said: "This is real life, not television or film. Mrs Turner is middle-aged, living in the suburbs - she's a wife and mother.
"She's never been in any trouble. She's a normal, law-abiding person.
"That morning, within an hour of her getting up, her house was full of paramedics and police and the horror of a lost life."
Miss Russell dismissed the prosecution claim that Mrs Turner smuggled out of the house a jacket which may have contained a confession written by her son.
She also reminded the jury that Mrs Turner had played no part in destroying the letter.
Her husband, Leigh Turner, 54, has been accused of using bleach to destroy an alleged confession letter from their son. He has been charged with perverting the course of justice.
Elliot Turner denies murder, but has pleaded guilty to perverting the course of justice.
Emily was found dead in his bed on May 7 last year.
The trial continues tomorrow, when the jury will be sent out.
- Solent News Agency