Opening the case for the prosecution at Oxford Crown Court, Oliver Saxby QC said: "The motive was financial gain- laced, as far as Ben Field is concerned, with a profound fascination in controlling and manipulating and humiliating and killing.
"The means were intricate - you will hear evidence of 'exit strategies' as he called them involving drugging and alcohol poisoning and suffocation whilst asleep or sedated and falls at home and attempts to cause heart failure, car crashes and unwitting overdoses."
He told the jury: "The common theme: Death made to look like accident or suicide - an elderly, ailing life coming to a sad but predictable end. That’s how they wanted it to seem. And why? Because, by then, he had deceived each into changing their will so he inherited their respective houses.
"If he was to inherit their houses, they had to die and if he was to enjoy his inheritance, he had to get away with it.
"For Mr Field, this was a project. Befriend a vulnerable individual, get them to change their will and then make sure they died. It is a project he seems to have relished devising and managing and executing - and, to an extent, documenting - in various notes and diaries he made.
"Indeed, piecing things together, it is clear that his project became his life's work - a life's work of which he was proud and for which he admired himself.
"Mr Farquhar did die. Field killed him, almost certainly by suffocating him. Field tried to kill Ms Moore-Martin. By a manner of means. But his 'exit strategy' for her was cut short because Ms Moore-Martin's niece became involved and Ms Moore-Martin survived - only to die a little later from natural causes."
Bespectacled and bearded Field stood in the dock alongside Martyn Smith, aged 32 years, who was also accused of one count of murder, one count of attempted murder and conspiracy to murder.
Both men deny the charges.
Mr Saxby said: "To carry out his grand design, Field needed help. And, in Smith, he found help - and encouragement.”
He added: "Smith lent his assistance and support - in part because, like Mr Field, he was greedy, in part because he got carried away in Field's world of plotting and deceit and death and in part because he was impressed by Field, somewhat in his thrall."
Also in the dock was Benjamin's brother Tom Field, who is 23. Wellingborough Road, Olney, Bucks, who is Benjamin Field's brother and he was charged with fraud by false representation.
The prosecutor told the jury this was in relation to an alleged incident where the two Field brothers and Smith convinced Ms Moore-Martin that Tom Field needed £27,000 to buy a kidney dialysis machine so he could remain at Cambridge university. He denies fraud.
The jury heard how Mr Farquhar was found dead by his house cleaner on October 26 2015 at his home in the village of Maids Moreton.
They were told told how “private” Mr Farquhar was, at heart, a lonely man, which was something Field was able to exploit in his plot to get his inheritance and murder him.
After the ‘betrothal ceremony’ with Field, Mr Farquhar had written: “It is one of the happiest moments of my life. Gone are the fears of dying alone.”
Mr Saxby revealed how the retired teacher and published novelist had been a guest lecturer at the University of Buckingham when he met Field and Smith, who had both become students there in 2011.
Field was studying English and Smith was also a student who worked as a magician. The two men moved in with Mr Farquhar.
Mr Saxby said: “For all that he (Mr Farquhar) was financially secure, for all that he travelled a fair amount, for all that he had a variety of interests, for all that he had a legion of friends from various quarters of his life, for all that he had close family - he seems essentially to have been a lonely man.
“Mr Farquhar was gay. A number of friends guessed this and a selected few were told. What was clear to all - whether they knew or guessed - is that he found his sexuality something very difficult to come to terms with, something with which he struggled - not least because he regarded it as incompatible with his religious beliefs.
“Field saw that Mr Farquhar was vulnerable and this was something, from the very outset, he decided to exploit.”
Mr Saxby said friends described Mr Farquhar, who owned the house he lived in, free of mortgage, and had various savings and other investments, as intelligent, deeply Christian, tough, someone it would take a lot to knock down. He added that he was fiercely private, clever...and innocent.
The jury heard that arrogant Field had kept multiple diaries where he had outlined his cynical view of older Peter Farquhar and had documented his campaign to break him down.
He was said to have described their relationship as “vulgarly transactional” and Field said he was “centered on career-minded avarice” because he wanted to work at Buckingham University or the Stowe School.
After police arrested Field, they recovered vast quantities of material in diaries, notes and electronic files that all referenced his grim projects his plans.
The court heard the two defendants lace Mr Farquhar's his food and drink with psychoactive drugs, some of which were legal to buy online
The alcohol and the psychoactive drugs combined to leave him a "dribbling shambles of his former self" who suffered hallucinations before his death
Field and Smith encouraged Mr Farquhar to drink more alcohol, although he was just a social drinker, so that it looked like his decline was all down to him becoming a sad alcoholic. They also encouraged him to commit suicide.
“In time, Mr Field and Mr Farquhar entered into a relationship together. There was the form of ‘marriage’ ceremony. They shared a bed. One or two of Mr Farquhar’s journal entries suggest the relationship became a sexual one, at least to a degree," said the prosecutor.
“In relation to Mr Farquhar, perhaps it does not matter whether it did or not - what is clear is that Mr Farquhar fell in love with Mr Field. Against which backdrop, with Mr Smith’s help and support, Mr Field got Mr Farquhar to change his will so that he and Smith became beneficiaries.
“Plus, they started to drug him. Using sedatives and other psychoactive drugs, including ‘new psychoactive drugs’ - in other words, man-made drugs available for purchase on the internet which the law - and forensic science - were struggling to keep pace with.
“Field and Smith were giving Mr Farquhar these drugs covertly. That is to say, without him knowing it - by putting them in his food and his drink, for instance. They were doing this on a very regular basis.
“Moreover, they were also encouraging Mr Farquhar to drink alcohol. Mr Farquhar was a social drinker but they encouraged him to drink more - whisky, especially.
“For Field and Smith there were two benefits to feeding Mr Farquhar drugs and getting him to drink a bit more alcohol. First, the one accentuated the effects of the other. Hallucinations, unsteadiness on his feet, slurring his words - in time, courtesy of the drugs and the alcohol, Mr Farquhar became a dribbling shambles of his former self.
“The second benefit of what they were doing? It looked like how he had become was all down to alcohol, to Mr Farquhar being some sort of sad alcoholic."
The prosecutor told the jury that Mr Farquhar had suffered from thoughts of suicide as he could not figure out what was causing his deteriorating mental state.
“How Mr Farquhar had become remained undiagnosed right up until his death, indeed for some time after his death. Because nobody knew they were drugging him. Least of all, Mr Farquhar.
“The agony Mr Farquhar lived through was not only the symptoms themselves but also the absence of a diagnosis, with all this entailed - the fear he might have dementia, the public perception he must be some sort of closet alcoholic.”
Mr Saxby argued that Mr Farquhar did not kill himself and actually began to improve after some time spent in a care home but was killed by Field on October 25 2015, with Smith’s help.
Field collected the inheritance of £20,000 and the property on Manor Park Road, Maids Moreton. Smith collected £10,000, the court heard.
Ben Field denies murdering Mr Farquhar and denies attempting to murder Ms Moore-Martin.
The jury heard Field admits poisoning Mr Farquhar but claims he only did it because the older man was difficult to live with.
Mr Saxby told them: “As to giving Mr Farquhar drugs without him knowing it - drugging him, Field accepts this, too. It was not done in furtherance of this fraud, he says, nor with the intention of killing him. Rather, he drugged him because Mr Farquhar was proving difficult to live with and that was a way of coping with the situation.
“He accepts defrauding him. Specifically, Field accepts pretending they were in a genuine relationship so as to get Mr Farquhar to change his will - which, of course, Mr Farquhar did.”
In addition, Field accepted defrauding Ms Moore-Martin by pretending to be in a relationship to get her to change her will and defrauding her by getting her to hand over money for a car and the money for Tom Field’s dialysis machine. He denies drugging her or at any stage wanting her dead.
He also accepts two counts of burglary.
Smith and Tom Field denied any wrongdoing.