Luke D’Wit, 34, who befriended and worked for Stephen and Carol Baxter, created a series of fake online personas to manipulate them before he killed them.
These included a doctor from Florida and members of a fake support group for the thyroid condition Hashimoto’s, which Mrs Baxter suffered from.
D’Wit, of West Mersea, arrived soon after and described himself as a “friend” to a 999 call handler, before calmly giving a false account.
Prosecutors said he created a fake will on his phone the day after the Baxters were found dead, making him a director of their shower mat company Cazsplash.
Another fake persona – a solicitor – was used in connection with the new will, prosecutors said.
Tracy Ayling KC said in her prosecution closing speech that D’Wit murdered Mr and Mrs Baxter “calmly, coolly and in a way which had been entirely planned, maybe for some while”.
He was found guilty at Chelmsford Crown Court of murdering the couple following a trial lasting more than a month.
The Baxters’ daughter Ellie said in evidence that her parents believed D’Wit was “weird, but nerdy weird”.
She said he had initially been brought into their shower mat business in about 2012 or 2013 to “help build the website” before eventually coming round to their house “every day”.
Detective Superintendent Rob Kirby, of the Kent and Essex Serious Crime Directorate, said in an interview outside court that D’Wit was “without doubt one of the most dangerous men I’ve ever experienced in my policing career”.
(D’Wit) befriended people, came across as a very amenable, helpful person but in the background he was a cool, calculated killer who spent years planning the demise of Carol and Stephen Baxter
Detective Superintendent Rob Kirby
He said: “I have absolutely no doubt that had he not been caught, he would have gone on to commit further murders.”
Mr Kirby said that “justice has been served today”, adding that D’Wit “rightly belongs behind bars”.
The defendant “fooled everyone”, he added.
“He befriended people, came across as a very amenable, helpful person but in the background he was a cool, calculated killer who spent years planning the demise of Carol and Stephen Baxter,” he said.
He described D’Wit as a “loner” who “spent hours of his time creating false personas, all there to create control over the Baxters”.
“The level of deviousness he went to was phenomenal,” Mr Kirby said.
Asked about a possible motive, Mr Kirby said it was “unclear what was going on in D’Wit’s mind”.
“Certainly he stood to benefit financially from the death of the Baxters and we believe that certainly this played part of the role in his motive,” he said.
He continued: “D’Wit’s downfall was the arrogance that existed within him.
“He didn’t cover his tracks properly and he was deluded in thinking that he could use fentanyl to kill two people and that wouldn’t be found to be suspicious.”
D’Wit was arrested at his workplace and his bag contained fentanyl patches, opened and unopened.
Prosecutors questioned his claim that he was taking these back to a pharmacy following the death of his father in 2021.
D’Wit denied murdering Mr and Mrs Baxter and claimed in court that he created fake identities on the instructions of Mr Baxter and to give Mrs Baxter “someone to talk to and air all her grievances to”.