Benjamin Field, 28, who killed Peter Farquhar, 69, in order to inherit his house and money was given a life term with a minimum term of 36 years by Mr Justice Sweeney at Oxford Crown Court on Friday.
Detectives branded Field a psychopath and said he would have posed an "ongoing danger to society" had he not been stopped.
Field secretly gave Mr Farquhar drugs and spiked his whisky, hoping that his eventual death at his hands would look like suicide or an accident.
It was only when the Baptist minister's son began targeting Mr Farquhar's neighbour, Ann Moore-Martin, in the village of Maids Moreton, Buckinghamshire, that his wicked scheme began to unravel.
The sexually promiscuous Field also manipulated Miss Moore-Martin, a deeply religious retired headteacher, by writing messages on her mirrors purporting to be from God.
He admitted fraudulently being in relationships with the pensioners as part of his plan to get them to change their wills.
Field accepted he had "psychologically manipulated" the retired teachers but denied any involvement in their deaths.
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 underwent a "betrothal" ceremony with gay Mr Farquhar while also having a string of girlfriends, and was in a sexual relationship with Miss Moore-Martin, who was 57 years his senior.
Prosecutors said Field had a "profound fascination in controlling and manipulating and humiliating and killing" and alleged he plotted his crimes with his friend, failed magician Martyn Smith, 33.
Field, a University of Buckingham PhD student, was a prolific writer, documenting his thoughts and activities in diaries and journals.
He had also drawn up a "100 clients" list, including his parents, grandparents and brother, which the prosecution said were future targets.
Field denied murdering the author and University of Buckingham lecturer and maintained Mr Farquhar could have died from taking his usual dose of flurazepam and drinking whisky.
He told the jury he had created fake relationships with Mr Farquhar and Ms Moore-Martin in order to benefit financially from their deaths but had not wished them dead.
Field, of Wellingborough Road, Olney, Buckinghamshire, previously pleaded guilty to defrauding Ms Moore-Martin of £4,000 to buy a car and £27,000 for a dialysis machine.
He also admitted two burglaries and the two fraudulent relationships.
Field insisted he had acted alone and not recruited his young brother Tom, 24, or Mr Smith to help with the dialysis fraud.
The jury found Field not guilty of conspiracy to murder Miss Moore-Martin and the alternative charge of attempted murder. He was also found not guilty of possession of an article for the use in fraud.
Mr Smith, of Penhalvean, Redruth, Cornwall was acquitted of murder, conspiracy to murder, possession of an article for the use in fraud, two charges of fraud and one of burglary.
Cambridge University graduate Tom Field, of Wellingborough Road, Olney, Buckinghamshire, was found not guilty of a single charge of fraud.
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