Experts who studied Edward Cairney and Avril Jones for a TV documentary to be shown tonight insisted their refusal to disclose Ms Fleming’s whereabouts was their way of taunting the police and their victim's loved ones.
Ms Fleming was 19 when she was last seen in December 1999, but police did not start a missing person investigation until October 2016 that a missing person investigation was launched.
Cairney, 79, and Avril Jones, 60, of Inverkip, Renfrewshire, were arrested in 2017 and convicted of her murder in July 2019.
Jones was also found guilty of fraudulently claiming £182,000 in benefits by pretending Ms Fleming was alive. Dr Mohammed Rahman, a criminologist at Birmingham City University, said: "When killers don't confess to the crimes they have committed, or tell the authorities where their victim's body is located, this for many is their last form of control.
"We know that Cairney is an individual who liked to be dominating and as such this is one of the last acts of control that he has over the authorities, over the victim and over the victim's family.
"Cairney was clearly an individual who was controlling and dominating. He always wanted to be in control of himself, his emotions and his surroundings. When he was unable to successfully do this, what he would tend to do is be confrontational and retaliate."
He added: "A person who is able to lie so elaborately about the killing of someone is clearly an individual who has depersonalised and dehumanised the victim. They consider the victim as an object and as such have been able to objectify their killing for just money."
In the documentary Murder at my Door, psychologist Anjula Mutanda said the killers' actions were prolonging the pain for their victim's friends and relatives and denying them closure.
She said: "What happened to Margaret is an unfinished, incomplete story and psychologically that can be deeply, deeply painful.
"Where is she? What happened? How did she die? We can't even bury her, we can't even honour the life that she had. All these questions remain questions for the rest of their lives until these two people decide whether they are going to speak out or not."
Jurors found that Cairney and his partner Jones murdered Ms Fleming between December 1999 and January 2000. They were jailed for life with 14-year minimum terms.
The authorities were alerted to Ms Fleming’s disappearance in 2016 when Jones applied for personal independence payments on her behalf. She wrote that Ms Fleming needed constant care and had self-harmed and been found eating from a dog bowl.
Her claims prompted social services and police to visit the home.
Murder at my Door: The Mystery of Missing Margaret Fleming will be shown on the Crime+Investigation channel at 9pm tonight.