An IT worker who poisoned a married couple with the opioid painkiller Fentanyl and rewrote their wills to take control of their shower mat company has been convicted of the two murders.

Luke D’Wit, 34, became friends with Stephen and Carol Baxter (61 and 64) while working for them and later told police he was “like an adopted son to both of them”.

Just one day after her death, on April 10 at 6:54 a.m., he created a will on his mobile phone, leaving him the couple’s company, Cazplash.

The description states that he is the “beneficiary of a very strange will” and the terms included that he would be given all business decisions as “a director and person with significant control.”

Although D’Wit, of Churchfields, West Mersea, denied the murders, he was found guilty on both counts.

The couple’s daughter found them dead in the conservatory of their home in Victory Road, Mersea Island, on Easter Sunday last year.

Both were sitting in their armchairs, had no suicide note, and their house and kitchen were clean and tidy.

In one emergency call, her daughter Ellie Baxter could be heard screaming and crying as she told the emergency operator, “I need an ambulance immediately,” before being heard banging on the glass of the conservatory.

The phone was then passed to D’Wit, who described himself as a “friend” and remained “very calm and credible” throughout the conversation.

Although he was not initially treated as a suspect, the investigation changed after firefighters ruled out carbon monoxide poisoning and a toxicology report in June indicated that fentanyl played a role in both deaths.

In both cases, stomach contents were analyzed and this “suggests, but does not conclusively prove, that the drug was taken orally.”

“It is difficult to imagine a scenario in which two people who were not prescribed fentanyl could inadvertently contaminate their food with this drug,” the prosecutor told jurors.

Before her death, D’Wit had created a series of false identities, including that of a Florida doctor who gave Ms Baxter medical advice “without any clinical basis.”

Posing as Andrea Bowden, he told his victim that “visiting family releases too many chemicals in the adrenal gland” and warned her to reduce regular contact with her loved ones.

In addition, he posed as a support group with false identities who also suffered from Hashimoto’s, Mrs Baxter’s thyroid disease, and as a lawyer in order to convince her family of the authenticity of the will he had drawn up.

It later emerged that D’Wit had also installed a “mobile security monitoring application” that allowed him to operate a camera in the couple’s living room and that he had watched them die.

Analysis of his phone revealed images of the two of them “in their armchairs” on the afternoon of April 7 last year. From then on, Mrs Baxter’s pacemaker stopped recording any further movements.

Jurors heard that D’Wit began working for the couple in 2012 or 2013, with Mrs Baxter becoming increasingly dependent on him due to her ill health.

Her daughter described him as “strange.” He knew the security key to the gate of her house, knew where a key safe was, and helped her mother administer her medication.

Officers also found a bag containing several metal nails and pill casings. We firmly believe that Carol had previously received these and that this led to her admission to hospital where x-rays showed the nails in her abdomen.

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 have ever encountered in my career as a police officer”.

He said: “I have not the slightest doubt that he would have committed further murders if he had not been caught.”

Mr Kirby said “justice has been done today”, adding that D’Wit “rightly belongs behind bars”.

The defendant had “made a fool of everyone,” he added.

“He made friends with people, seemed like a very friendly, helpful person, but behind the scenes he was a cool, calculating killer who spent years planning the deaths of Carol and Stephen Baxter,” he said.

He described D’Wit as a “loner” who “spent hours creating false identities just to exert control over the Baxters.”

He continued: “D’Wit’s downfall was the arrogance that was in him.

“He didn’t cover his tracks properly and was under the delusion that he could kill two people with fentanyl without it being considered suspicious.”

He is due to return to Chelmsford Crown Court on Friday for sentencing.