Charles O'Neill and William Lauchlan are not unknown in the Scottish courts and were once described by a judge as "evil, determined and manipulative".
They are not only killers, but predatory paedophiles who have convictions dating back 12 years.
These include crimes such as serious sexual assault and raping children.
As the pair begin life sentences for killing Allison McGarrigle their previous dealings with the Scottish legal system can be revealed.
O'Neill and Lauchlan, who were partners, killed Mrs McGarrigle in 1997 to stop her reporting the abuse of a young boy.
They then put the 39-year-old's body in a bin and disposed of it at sea.
Mrs McGarrigle was reported missing in February 1998. Her body has never been recovered and she was declared dead in 2005.
Although it would be 13 years between the murder and O'Neill and Lauchlan being found guilty, the pair were convicted of other offences in between.
In 1998, O'Neill and Lauchlan were jailed for eight years and six years respectively for 31 charges of sexually abusing children.
They lured five young boys between the ages of nine and 15 to a house in Skelmorlie, North Ayrshire, where they fed them drugs and alcohol before molesting them.
The pair were released early in 2002. Lauchlan went to Spain and was joined there in 2003 by O'Neill.
While he was still in Scotland following release from prison, O'Neill befriended a family in Irvine and earned their trust before sexually abusing a 14-year-old boy.
After committing this offence in June 2003, O'Neill joined Lauchlan in Spain where they set up a window cleaning business.
They then befriended a family in a karaoke bar near Benidorm and lured two English brothers, aged 15 and 18, on a camping trip under the pretext of giving them a job.
When the older brother left, O'Neill and Lauchlan abducted the 15-year-old for three nights in April 2004 and attempted to drug and sexually assault him.
They were arrested but not prosecuted for the attack and were later deported to Scotland where they were returned to jail for breaching parole.
The pair were first charged with Mrs McGarrigle's murder in April 2005 - the year she was declared dead - but were not indicted after prosecutors considered the evidence.
On release from prison in 2007, they set up home in Blackpool and had to register with police as convicted sex offenders.
Lancashire Police were so concerned about the risk they posed that they ordered around-the-clock surveillance and obtained consent to place covert surveillance devices in the pair's van.
An intercepted call alerted police that O'Neill and Lauchlan were grooming a six-year-old boy in Scotland.
Over a six-week period they texted the child and his mother about 2,000 times and even bought the boy a mobile phone.
Central Scotland Police took over the surveillance operation when they travelled to Falkirk with the boy and his mother to stay at a hotel in 2008.
When officers searched the bedside cabinet they found a pair of the child's underpants and autobiographical stories of sexual abuse.
After O'Neill and Lauchlan were arrested, it emerged that they were planning to buy a camper van and take the mother and son to Spain under the promise of work.
They had also persuaded the woman to become a surrogate mother and give them a child.
While they were in custody earlier this year, prosecutors decided to charge O'Neill and Lauchlan with Mrs McGarrigle's murder.
Before the case came to trial, the pair were convicted at the High Court in Glasgow of the abuse charges relating to the 14-year-old boy in Irvine, the 15-year-old boy in Spain and of grooming the six-year-old in Scotland.
Trial judge Lord Pentland described the pair as: "Evil, determined and manipulative paedophiles of the worst sort".
These convictions could not be mentioned until now due to reporting restrictions imposed during the McGarrigle case.
Even now investigators are unable to determine whether they have traced all of O'Neill and Lauchlan's victims.
In Rothesay, on the Isle of Bute, the pair are known to have babysat for mothers who were heavy drinkers, allowing them access to their sons.
Among their friends at this time was Mrs McGarrigle.
She had been due to give evidence against them at an upcoming trial when she went missing in June 1997 after separating from her husband.
For months her benefits book lay unused. It was later found at the flat in Rothesay where she had moved with a young boy only a week before she disappeared.
Police believe she argued with O'Neill and Lauchlan because they knew she was due to testify against them.
It is thought they strangled her, hid her body on Largs beach before disposing of it in the Clyde.
The murder went unpunished for 13 years until police and prosecutors finally caught up with O'Neill and Lauchlan.