Thomas Inglis suffered severe head injuries in 2007
The judge trying a mother accused of murdering her brain-damaged son has said no-one has the "unfettered right" to take the law into their own hands.
Frances Inglis, 57, of Dagenham, Essex, denies murdering Thomas Inglis, 22, on 21 November 2008 and an earlier attempt to kill him on 4 September 2007.
Mr Inglis was fatally injected with heroin at his Hertfordshire care home.
Judge Brian Barker told the Old Bailey jury "there is no concept in law of mercy killing" and it is still killing.
He said the jury must "put all emotion aside" when considering the case of Ms Inglis, who gave her son a lethal heroin injection.
"People hold strong views all across the spectrum but we are not debating it here. We are not here to send a message," he said.
"Our laws are designed to protect us all within society and particularly to protect the weak."
Frances Inglis said she wanted her son to have a painless death
He added: "We must make it clear that there is no concept in law of mercy killing. It is still a killing no matter how kind the intention."
Judge Barker, the Common Serjeant of London, said there was no doubt Ms Inglis had tried to take her son's life in September 2007 and succeeded using an "identical" method 14 months later.
He said it was for the jury to decide whether she had "criminal responsibility for her actions on both occasions".
Mr Inglis suffered severe head injuries when he fell out of an ambulance in July 2007.
Summing up the case, the judge said the background of "tragedy and grief" will have struck a chord with all who had heard it.
He said: "It would be extraordinary if we didn't feel empathy with the family and what Mrs Inglis had to face."
After the summing up, the jury was sent out to start its deliberations.