A former deputy church warden could not have have legally caused a university lecturer's death by giving him potentially sedated whisky, the Supreme Court has been told.
Benjamin Field, 35, was jailed for at least 36 years in 2019 after being found guilty of murdering 69-year-old Peter Farquhar in Maids Moreton, Buckinghamshire.
In April, appeal judges quashed Field's conviction and ordered a retrial, but said the Crown Prosecution Service (CPS) could take the case to the Supreme Court first.
And on Tuesday, five justices at the UK's highest court were asked to consider whether Field could have made Mr Farquhar cause his own death without Field legally being responsible.
During his murder trial prosecutors said Field drove Farquhar to believe he was losing his mind, secretly drugging him and spiking his whisky so his death would appear to be suicide or an accident, allowing Field to inherit his home and money.
The case has previously been adapted into Bafta winning BBC drama The Sixth Commandment, starring Timothy Spall and Anne Reid.
Before his murder trial, Field admitted burglary and fraud offences connected with relationships he had formed with Farquhar and his neighbour, fellow pensioner Ann Moore-Martin, to persuade them to change their wills.
He received a concurrent 16-year sentence for those offences. He was cleared of conspiring or attempting to murder Moore-Martin and of possessing an article for use in fraud.
The hearing was due to conclude on Tuesday, with a decision expected at a later date.
David Jeremy KC, for Field who is opposing the appeal, said in written submissions that "the act of giving whisky and/or Dalmane was not capable of being a legally effective cause of death".
"It must follow that if Ben Field, possessed of a continuous intent to kill over many years, had handed Peter Farquhar his car keys at his request, and Peter Farquhar had driven off and died in a traffic accident, then Ben Field would have caused Peter Farquhar's death and been guilty of murder," he said.
Jeremy added: "Peter Farquhar had a choice whether or not to drink whisky given to him by Ben Field.
"He chose to do so freely and voluntarily, knowing what it was and knowing what he was doing."
The Court of Appeal previously heard there was no direct evidence of how the Dalmane was administered and that a post-mortem examination found a therapeutic dose.
Jeremy told the Supreme Court: "There was no reason why, if Ben Field had smuggled Dalmane into Peter Farquhar's food or drink that night, he would have restricted himself to a therapeutic dose."
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