Frances Inglis was convicted of murdering her son in January.
The eldest son of a woman convicted of murdering her brain-damaged child has called for changes in the law governing "mercy killing".
Frances Inglis, 57, injected a lethal shot of heroin into her 22-year-old son Thomas in November 2008 after an earlier attempt had failed.
In an interview on BBC 5Live, Alex Inglis said his mother had acted in Thomas' "best interest".
He said that the way the law deals with such cases should be changed.
Mrs Inglis, from Dagenham, was given a life sentence and told she must spend a minimum of nine years in prison when she was jailed in January.
Thomas died at a care home in Sawbridgeworth, Hertfordshire, after suffering serious brain damage when he fell from an ambulance in July 2007.
At her trial, his mother described his existence as a "living hell".
Speaking on BBC 5Live, Mr Inglis said: "It was her main fear that Tom was in constant agony and she felt that she had to release Tom from that."
Thomas Inglis suffered severe head injuries in 2007
He explained that the family and Thomas' doctors were discussing the possibility of applying to the courts to have food and water withdrawn from him to end his life but that he believed the process was "cruel".
He said his mother had acted to spare Thomas from being "dehydrated to death".
He also questioned why a legal, painless alternative to withdrawing nutrition could not be used.
"The legal alternative is so horrific that I really don't think it should be used... there are ways people can die peacefully and calmly, there are ways it can be done," Mr Inglis said.
He said that he would have considered ending Thomas' life himself if his mother had not done so and suggested that the law was failing to address the issue of mercy killing.
"I really think that people need to consider what's morally right before they do what the law says," he said.
"If it really comes to it, they should disregard the law because the law is not keeping up with modern society and modern medical techniques."