She said: "It is all about his human rights but what about my human rights? What about Becky's human rights? We are living in a nanny state, which protects killers and ignores victims. It is sickening."
The unprecedented case unfolded after taxi driver and father of three Halliwell became the prime suspect in the disappearance of Miss O'Callaghan in March last year.
The office administrator from Swindon, who had recently moved in with her boyfriend, Kevine Reape, 25, had been on a night out with friends and had just left a nightclub in the town when she went missing.
A huge search involving hundreds of local volunteers failed to find any sign of her but Halliwell came under suspicion after police spotted his taxi on CCTV in the area where Miss O'Callaghan was last seen.
The senior investigating officer in the case Detective Superintendent Steve Fulcher decided to put Halliwell under surveillance in the hope that he might lead them to the missing girl.
But the following day with time running out Halliwell was arrested in a supermarket car park on suspicion of kidnap.
Usually a suspect would be immediately taken to a police station where they would be questioned in the presence of a lawyer according to a strict set of guidelines.
However believing Miss O'Callaghan's life was in immediate danger DS Fulcher authorised a so-called "urgent interview" without a lawyer present. Halliwell initially failed to cooperate but when DS Fulcher drove him to remote Barbary Castle in Wiltshire and again urged him to confess, he took him to the makeshift grave of his victim close to Uffington in Oxfordshire.
Then in a stunning development, the former bin man, turned to the officer and said: "Do you want another one?".
He then took the officer to the exact spot in Gloucestershire where he had buried Miss Godden-Edwards' body almost a decade earlier.
He was subsequently charged with both murders, but within weeks his lawyers had applied to have both cases thrown out on the grounds that DS Fulcher had not followed the rules regarding questioning a suspect.
His lawyer, Richard Latham QC also claimed his client could not receive a fair trial because of prejudicial pre-trial publicity.
He said the breach of PACE amounted to an "assault on the integrity of the legal system".
The judge in the case Mrs Justice Cox dismissed the application regarding pre-trial publicity but ruled that all the evidence related to Halliwell's confessions and the discovery of the bodies was inadmissible.
Fortunately police had enough forensic evidence linking Halliwell to Miss O'Callaghan's death to charge him with murder and he pleaded guilty today.
However due to the passage of time there was no remaining evidence connecting him to Miss Godden-Edwards' and the case was unable to proceed.
DS Fulcher has been suspended while the Independent Police Complaints Commission (IPCC) carry out two investigations into his actions.
Appearing at Bristol Crown Court earlier this year DS Fulcher defended his actions.
He told the court: "I did think it was utterly ridiculous that someone who took me, 12 people and a surveillance helicopter to the deposition sites of two bodies would then seek to find some loophole or quirk in the law to get away from the fact that he was a multiple murderer."
The experienced officer - who was backed by colleagues and superiors in court - said he had arranged to meet Halliwell at Barbury Castle in Wiltshire in a desperate bid to save Sian.
He said: "It was my responsibility as the SIO responsible for Sian, for her life or her death. The last thing I could do is speak to him in person and that's what I did.
"The sole purpose was to seek to appeal to him to show me where Sian was with the intention of saving her life."
He added: "Her right to life was more important than having a basic right to interview of his. If he refused there was nothing else I could have done."
Miss Godden-Edwards' family have said they do not blame him in anyway for what has happened.
Mrs Edwards said: "Steve Fulcher is a committed and dedicated police officer and a family man. As far as I am concerned he is a hero and I just want to give him a big hug.
"He knew that if he took Halliwell back to the police station and allowed him to see a lawyer, any chance of him owning up to what he had done would be gone. We would probably have never found Sian and I would still not know where my daughter was.
"He has lost his job for doing his job and it is an absolute disgrace."
Last night Detective Chief Superintendent Keir Pritchard of Wiltshire Police said the investigation into Miss Godden-Edwards's death was still live but conceded that there was no fresh evidence available to move the case on at this stage.
He said: "This very much remains a live investigation. I have a fresh senior investigating officer and we will pursue all the lines of inquiry vigorously.
"We will continue to give support to the families and we will be looking at every single line of inquiry"
"The most likely compelling evidence would be if we had an admissible confession to someone else."
Police have also said they are looking at other missing persons cases to see if there could be any link with Halliwell.
Chillingly just before his arrest he told his colleagues at the taxi firm: "Who knows what or who you find buried out there, there could be loads of people over the years."
