Witnessing the ordeal of a woman who killed her abusive husband with a hammer was "like seeing a curtain fall", her son has said.
Sally Challen is a free woman after her murder retrial was dropped on Friday.
Her son, David, said his father, Richard, dominated her life in "one of the worst cases of coercive control".
His 65-year-old mother, from Surrey, who met her husband at the age of 15, was still taking her first steps in a world without him, he said.
On Friday, prosecutors accepted Mrs Challen's manslaughter plea after her murder conviction was quashed in February.
She had been due to face a retrial at the Old Bailey but was instead sentenced for manslaughter and walked free, due to the amount of time she has already served in prison.
Mr Challen said the law "shouldn't have painted her as a cold, calculating murderer, as it does with every woman that kills".
He said there had been overwhelming evidence of his father's controlling behaviour, but coercive control only became a criminal offence in 2015.
"It's gradual," he said. "It's seeing your mother in a loving relationship as a child, and then growing up and seeing that curtain fall."
He said he watched his father become more cavalier and more open with his controlling actions over the years, restricting where his mother went, depriving her of freedom of thought and taking people out of her life.
"He controlled the parameters of her life - every bit of it - for 40 years," he said.
After Friday's hearing ended, Mrs Challen told reporters she still loved and missed her husband, but her son described her words as evidence of her dependency.
He said his father was "just playing games" - including telling his mother she was going mad when she confronted him with evidence of his serial infidelity.
His father also promised to resume their relationship if she signed a post-nuptial agreement, but then backtracked from it, he added.
The couple had been trying to reconcile in 2010 when she killed him at their marital home in Claygate.
Mr Challen said he took his father to task many times but it was "like nailing jelly to a wall".
He said health workers should had intervened on his mother's behalf as soon as signs of control were spotted - and had coercive control been an offence at the first trial in 2011, the situation would have been different.
"We would have heard all the evidence, not through a skewed vision of 'this isn't abuse, this is just a toxic relationship'," he said. "We would have understood it."