The Soham trial judge has warned jurors
that Maxine Carr's anger with Ian Huntley may have coloured any evidence against her former boyfriend.
The judge warned against "trial by media"
Judge Justice Moses said she clearly had an "axe to grind" with him.
Ian Huntley denies murdering Holly Wells and Jessica Chapman, but he admits they died at his home and he disposed of their bodies.
Ms Carr denies conspiring to pervert the course of justice and two counts of assisting an offender.
The judge was summing up before sending the jury out to consider its verdict on Friday.
He reminded them of Ms Carr's outburst in the witness box in which she "exhibited considerable anger against him".
He said: "I am not seeking in any way to cast doubt on the truth of her evidence, it is for you to judge that, but she did give evidence in respect to
important features which touched upon his case.
"You have to be careful about it, she has an axe to grind."
While there was "no dispute" about Ms Carr lying to police, the judge said the jury had to believe she knew Mr Huntley was guilty, in order to convict her of assisting an offender.
Considering whether she conspired to pervert the course of justice, he said: "If she intended to put the police off questioning or even suspecting
Huntley, whatever her motive for doing so, then she is guilty.
"You may think she came very close to admitting that in the very final stages
of her cross examination by Mr Latham."
Mr Justice Moses also urged the jurors not to be influenced by the press or emotions roused by the case and to judge it on the evidence alone.
He told them it was a tragic case and some of the facts were bound to provoke a reaction.
But he warned them the fact Mr Huntley had dumped the girls' bodies in a ditch did not mean he murdered them.
"Any emotional reaction to the events cannot and must not influence your verdict," he said.
Mr Justice Moses explained the prosecution did not have to prove a motive, but it did have to prove the guilt of one or both defendants.
He also told the jurors to discount a previous rape allegation against Mr Huntley which was later dropped, because it had nothing to do with the case.
Mr Huntley has told the trial Holly died accidentally, after falling into his bath as he helped her with a nosebleed.
But he admitted killing Jessica by putting his hand over her mouth to stop her screaming.
The judge said jurors should judge Jessica's death in the context of how Holly died.
If they thought Mr Huntley's account was "plainly unbelievable" then they might think he was lying about Jessica's death.
If they believed his story, he is guilty of Jessica's manslaughter, but not murder, the jury was told.
But if jurors believe he had intended to kill her or cause her serious harm, he should be found guilty of murder, he said.
Duty of care
If they did not believe Mr Huntley's story about Holly apparently drowning, they should find him guilty of murder.
But if jurors believed him, they would have to find him not guilty of murder or
The judge said Mr Huntley's lawyer had put forward another alternative, of "manslaughter by gross negligence".
This meant he had a duty of care towards Holly and failed to act to prevent her death, like a schoolteacher who allowed pupils to put themselves in danger.
The girls went missing in August last year
Mr Justice Moses then turned to the evidence concerning the events of 4 August 2002, the night the girls died.
He questioned why Jessica's mobile phone had been turned off within 15 minutes of the girls entering Mr Huntley's house.
He highlighted the fact Mr Huntley had said Holly's nosebleed had not stained her shirt because there was not much blood.
If so, why was it necessary for him to take the girls upstairs, asked the judge.
After completing his summing up of the prosecution case, the judge then started to remind the jury of the evidence used by the defence.
Earlier on Thursday, Michael Hubbard QC made his closing speech in defence of Ms Carr, who he said was vilified in prison.
He said there was an "incalculably evil" side to Mr Huntley of which his client had been unaware.
The jury is expected to be sent out on Friday.