Jurors at the Joanna Yeates murder trial retired today to consider their verdict.
The judge urged the six men and six women trying Vincent Tabak to reach a unanimous verdict.
Jurors at Bristol Crown Court "should not allow emotion or sympathy" to cloud their judgment in deciding whether Tabak intended to kill Miss Yeates, Mr Justice Field said.
The defendant, 33, denies murder but admits the manslaughter of the 25-year-old at her Clifton flat.
The judge told the jury: "It is your responsibility, and your responsibility alone, to judge the evidence and decide all the relevant facts, and that is a heavy responsibility.
"The defendant is charged with murder - the most serious charge in the calendar of the criminal law.
"Please do not allow emotion to enter into your deliberation.
"This is a tragic case. A lovely young woman, with a promising future ahead of her, has been robbed of her life.
"Her death will have, and doubtless continues to have, a devastating effect on her family and Greg.
"You must not allow emotion or sympathy for Joanna and her family and for Greg to cloud your judgment."
Mr Justice Field said the jury needed to focus on what was Tabak's intention at the time the landscape architect died.
"Did he intend to kill her or cause her really serious harm?," the judge asked.
"The fact that afterwards the defendant may have regretted what he had done does not amount to a defence.
"If having examined the evidence, and despite the defendant's denial, you are sure that when the defendant strangled Joanna Yeates he intended to kill her or cause her really serious bodily harm, your verdict will be guilty.
"If you are not sure of his intentions when he strangled Joanna Yeates your verdict should be not guilty."
Mr Justice Field carefully recounted the evidence taking the jury through Tabak's oral evidence and his account of his movements on the night of December 17.
He also spoke of the evidence given by pathologists Dr Russell Delaney and Dr Cary.
The judge said that of the 43 injuries on Miss Yeates's body some could have been caused from the same impact.
He also said that the jury should put to one side the injuries classed as abrasions as they could have been caused after death.
Instead they should focus on the bruises to Miss Yeates's body as they could only have been caused in life.
The judge said jurors should carefully weigh up the evidence of the two pathologists as to the time it took for Miss Yeates to die and the degree of force used.
Mr Justice Field said another factor for the jury to consider was at what time Tabak strangled Miss Yeates.
Mr and Mrs Lehman, who were walking to a party in Canynge Road, gave evidence as to hearing a scream, a two second pause and then another muffled scream coming from the direction of No 44 at around 8.45pm.
Other residents living nearby also heard it but a neighbour of Miss Yeates's, also living at No 44, did not, the judge told the jurors.
Tabak maintained in evidence that he did not kill Miss Yeates until after 9.25pm.
"It is a matter for you to consider whether you are satisfied that these two screams that the Lehmans heard were Joanna's screams," the judge said.
"If you are, this attack occurred well before the time that Vincent Tabak gave you and it would have been over before he sent the first text at 9.25pm to Tanja Morson in which he said he was bored."
Mr Justice Field told the jurors to exercise "care and caution" in considering the lies Tabak admits telling.
"You have heard that he accepts he cynically deceived Tanja Morson and he was very busy on the internet," the judge said.
"The defendant admits he told a series of calculated lies to the police."
The judge said Tabak's self-confessed lying could assist the jury in assessing the evidence.
"First, the fact he lied in these very important matters may affect your assessment of him as to whether he is a truthful witness," he told the jurors.
"It does not necessarily follow that because he told lies he admits to he has lied to you in the witness box.
"It is a matter you are invited to take into account."
The judge said it was up to the jury to decide whether Tabak's lies supported the prosecution assertion that he murdered Miss Yeates.
"However, you must not assume that because he lied he must be guilty," the judge said.
"You must consider why he lied.
"He told you that he lied because he wanted to conceal the fact he had killed Joanna.
"What does that tell you about the intention he had when he killed Joanna?
"It will only be that if you are sure he lied, not only to conceal his guilt of strangling Joanna but also to conceal that he intended to kill her or cause her really serious physical harm, that his lies can be taken into account as to supporting the prosecution's case that he had the necessary intention to murder."