Dad whose daughter killed her own mother says he "understands" the twisted motive.

It was the murder case that had the whole world talking: Missouri woman Dee Dee Blanchard was stabbed to death in her own home in June 2015, and her sick, wheelchair-bound daughter was responsible.

The then 24-year-old Gypsy Rose had engaged her partner, a man she had met on a Catholic dating website two years earlier, to kill her mother and free her from her mother’s control.

Watch the trailer for Mommy Dead and Dearest. Post continues after video.

As the investigation into Dee Dee’s death unfolded, the public learned the horrific truth: that Gypsy Rose wasn’t sick and had actually never been ill at all.

For her whole life, the world had been led to believe that Gypsy was confined to a wheelchair due to muscular dystrophy and paraplegia, that she had developmental delays and was even undergoing treatment for leukaemia.

But her illness was an elaborate lie made up by her mother Dee Dee in what is considered one of the most severe cases of Munchhausen by Proxy Syndrome the world has ever known.

Now, as a documentary called Mommy Dead and Dearest about the shocking crime is due to be released, Gypsy's father Rod Blanchard says he "understands" why Gypsy turned to murder in order to escape her mother's sociopathic prison.

"Yeah, it's wrong, and there were other options that Gypsy had," Rod told PEOPLE Magazine.

"[But] she tried some of those avenues and they didn't work out.

"Bad as it sounds, she [Dee Dee] asked for what she got."

Speaking to the New York Post Rod added that if his ex-wife was still alive he "might do the same thing Gypsy did".

"It's absolutely horrifying that you can do that to your child," he said.

Rod said he and Dee Dee had separated before Gypsy's birth, and that he rarely saw his daughter because he was told he "wasn't equipped" to handle her health issues and care.

Rod said his ex-wife's stories, and Gypsy's acting, was so convincing, he really believed that his daughter was facing the fight of her life.

"I really, genuinely believed that she had some issues. Dee Dee was really good at convincing me that there was a problem," he told the New York Post.

"I never, ever can fathom that she was really in the hospitals, and the doctors wouldn't have caught on to something at some point over the years."

Rod would even send Dee Dee and Gypsy extra money to help pay for the vacations they took to take a break from her gruelling treatment.

Gypsy is currently serving time behind bars, after she was sentenced to the minimum 10 years in jail after she plead guilty to second-degree murder in July 2016.

Her boyfriend, Nicholas Godejohn, is still awaiting trial. If found guilty, he faces life in prison.

LISTEN: Mamamia's True Crime podcast discusses the murder of Sallie-Anne Huckstepp.

Rod and his new wife, Kristy, say they will "always have a home" for her when she is eventually released.

"We'll put her on the right path," he said.

Mommy Dead and Dearest is due for release in the US on May 15.