Benjamin Field (above right), 28, was found guilty at Oxford Crown Court of the murder of Peter Farquhar (above left), 69, in the village of Maids Moreton, Buckinghamshire.
His co-accused, magician Martyn Smith, 32, was found not guilty.
The pair were both cleared of a charge of conspiracy to murder Mr Farquhar's close neighbour Ann Moore-Martin and Field was also acquitted of her attempted murder.
Field killed Mr Farquhar by making his deaths look like an accident or suicide.
Field, a Baptist minister's son, admitted fraudulently being in relationships with Mr Farquhar and his neighbour Miss Moore-Martin as part of a plot to get them to change their wills but denied any involvement in their deaths.
Prosecutors told Oxford Crown Court Field and Smith targeted Miss Moore-Martin a few months after murdering Mr Farquhar but they were acquitted of charges relating to her death after 77 hours of deliberations.
Field spiked Mr Farquhar's food and drink with neat alcohol and hallucinogenic drugs as part of the gaslighting, while Field wrote messages on Miss Moore-Martin's mirrors purporting to be from God.
Mr Farquhar, who was torn about his sexuality because of his religion, died in October 2015, while Miss Moore-Martin died in May 2017 from natural causes.
Field had undergone a "betrothal" ceremony with gay Mr Farquhar while also having a string of girlfriends and was in a sexual relationship with spinster Miss Moore-Martin, who was 57 years his senior.
Prosecutors said Field had a "profound fascination in controlling and manipulating and humiliating and killing".
The PhD student was a prolific writer, documenting his thoughts and activities in diaries and journals.
Field had also drawn up a "100 clients" list, including his parents, grandparents and brother, which the prosecution said were future targets.
He denied murdering author and University of Buckingham lecturer Mr Farquhar and maintained he could have died from taking his usual dose of flurazepam and drinking whisky.
"This murder never happened. No-one killed anyone," he told jurors.
Senior investigating officer Mark Glover, a retired detective chief inspector, said Field fitted the profile of a psychopath.
"Cruel, calculating, manipulative, deceitful. I don't think evil is too strong a word for him," he said.
Mr Glover said Field had taken pleasure in tormenting his victim and torturing him physically and mentally, adding: "Everything is about Ben Field and Ben Field's gain."
Chris Derrick, head of complex casework unit at CPS Thames-Chiltern, added: "I think torture is a word that can be used to described Benjamin Fields' behaviour."
He added: "He is clearly a very calculating and ruthless man who spent a great deal of time planning what he was going to do."
Field's brother Tom, 24, a Cambridge University graduate, was cleared of a single charge of fraud.
Benjamin Field showed no emotion as the jury forewoman returned the verdicts.
When his brother, who was on bail, was released from the dock he hugged his parents.
Mr Justice Sweeney adjourned sentencing against Field until a date to be fixed after ordering a pre-sentence psychiatric report.
He was remanded into custody until then.
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