Ian Huntley's defence says there is evidence he is guilty of Holly Wells and Jessica Chapman's manslaughter.
The prosecution says Mr Huntley killed the girls
Stephen Coward told jurors Mr Huntley would not claim to be innocent of all responsibility and should be punished.
But the QC denied a "sinister" sexual motive and said the evidence suggested it was an innocent accident.
Former school caretaker Mr Huntley denies murdering the 10 year olds last year, but admits they died in his house in Soham, Cambridgeshire.
Mr Coward told the Old Bailey on Wednesday: "We conclude on the evidence available, thin as it is, that there is evidence that Mr Huntley is guilty of the manslaughter both of Holly Wells and of Jessica Chapman."
The prosecution's suggestion that Mr Huntley had a sexual motive in inviting the pair in to his house was "a classic case of the prosecution looking through twisted
"They treated it as sinister from the start and we submit there is evidence, and the only evidence there is, which suggests it was entirely innocent from the start," Mr Coward said.
Mr Coward accused the other defendant Maxine Carr of "character assassination" as she tried to distance herself from her ex-boyfriend.
Ms Carr denies conspiring to pervert the course of justice and two counts of assisting an offender.
She was no "shrinking violet" but a "mistress of her own mind" who told police it was her idea to provide Mr Huntley with an alibi - but later changed her story in court.
He also said that Ms Carr's legal team had not challenged Mr Huntley's testimony, where he said he had changed the carpet in his car boot months before the girls died - not
while Ms Carr was in Grimsby, as she later told court.
Earlier, prosecuting counsel Richard Latham QC described Mr Huntley as a ruthless liar who killed the two girls.
But Mr Coward asked jurors to stick together, disregarding outside influences when making their decision on a case that had gone "deep into the psyche of everyone in Britain".
"Pressure has been exerted not just in the court," he said, because of daily media coverage.
"We have four naturally deeply distressed parents. We have two girls aged 10 who were alive and happy and are now dead.
"Inevitably, that imposes great pressure on everyone concerned in the case."
Mr Coward said in many vital areas, such as how and when the girls died, there were many "blank spaces".
And he warned jurors it was easy to take the moral high ground with the benefit of hindsight.
But some people would always panic, lack courage and would end up lying and trying to cover their tracks, he said.
"They are not always necessarily bad people, evil people or callous people," said Mr Coward.
"They were weak, they were foolish and they have dug a hole for themselves."
Holly Wells and Jessica Chapman died in August 2002
He said hindsight could also be applied to witness statements which were only taken after Mr Huntley and Ms Carr were arrested.
Some witnesses would naturally be trying to remember things that would help the prosecution case, he said.
His closing speech follows the prosecution's summing up earlier on Wednesday.
During his evidence Mr Huntley claimed Holly died accidentally after falling into his bath last August when he was helping her with a nosebleed.
However, he admitted he had killed Jessica by putting his hand over her mouth to stop her screaming and that he had told lies to police to cover his tracks.
He also admitted dumping the girls' bodies in a ditch and trying to burn them.
Mr Latham, said "ruthless" Mr Huntley's account of both deaths were "desperate lies".
He said "the whole business in the house was motivated by something sexual" that went wrong.
The case was adjourned until Thursday.