"I thank God he did it," Sandra Rzucek told ABC News of Chris Watts' decision to lie in TV interviews about what happened to wife Shanann and their girls

At first, it was the exact opposite of what they wanted: On Aug. 14 Chris Watts went on the local news to talk about his wife, Shanann Watts, and their two young daughters — all of whom had vanished the day before from the family’s home in Frederick, Colorado.

Shanann’s mother, Sandra Rzucek, warned him against just such a public appearance.

In hindsight, however, she’s glad he didn’t take her advice.

“I said, ‘Chris, I don’t think you should do any media,’ ” Rzucek recalled in an interview with ABC News airing Friday night on 20/20. “I said, ‘You’re the last one to have seen them, so I don’t think you should.’ ”

Speaking with multiple TV reporters in August, Chris, 33, pleaded for the return of 34-year-old Shanann, his wife of nearly six years, and girls Bella and Celeste.

“If somebody has her and they’re not safe, I want them back now,” Chris told local station KMGH. “That’s what is in my head. If they’re safe right now, they’re going to come back. But if they’re not safe right now, that’s the not-knowing part.”

Chris’ comments helped the case quickly attract even wider attention — and would later undercut his own explanation about what happened to his wife and kids.

He was arrested late the following day and accused of murdering his young family. In an interview with police, he claimed he killed Shanann in a rage when she strangled their youngest daughter in apparent retaliation over his desire to separate.

But if Chris’ confession was truthful, that he was provoked to deadly violence while defending his kids, then why later lie to reporters about his family’s whereabouts?

“I thank God he did it,” Rzucek told ABC News. “I thank God in heaven that he didn’t listen to me.”

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In November, Chris admitted to all of the charges against him in exchange for being spared a possible death sentence. In pleading guilty, he confirmed his earlier confession about the deaths was a ruse.

Prosecutors said in court he strangled Shanann and smothered their daughters, motivated in part by a desire for a “fresh start” with his mistress.

Shanann’s parents — dad Frank Rzucek and mom Sandra — jointly spoke out on 20/20 about the triple murder in their first-ever interview about the case.

“That’s the big word: why?” Frank told ABC. “He must have snapped, because there’s nothing else I could figure out what happened to him.”

“The Rzuceks are utterly devastated,” 20/20 co-anchor Amy Robach tells PEOPLE. “They told me that in one night they lost all their grandchildren and their only daughter, a daughter they video chatted with every single day.”

Robach says Frank and Sandra have “donated all of their daughter’s and granddaughters’ possessions to those in need and they are raising money in honor of Shanann and her children to donate to charity.”

“They want everyone to know what a devoted mother she was to her children, what a wonderful daughter and friend she was and what a positive force she was in the world,” Robach says. “They want to counter the version of events Chris told police and make it known she would never ever harm her children.”

As part of those ongoing efforts, the Rzuceks have sued Chris for wrongful death in the three murders, asking for compensation to make up funeral costs, Shanann’s future income and for emotional distress, ABC News reports.

“What we’re concerned about is somebody is going to come and offer him a lot of money to tell the world … at least a version of what he contends happened that night,” Rzucek attorney Mike Newman said on 20/20. “We’re not going to allow him to do that. So, that’s why we filed the wrongful death suit.”

Prosecutors have requested the court order Chris to pay nearly $50,000 in restitution but noted that, with some pending claims against him, that number could increase.

Despite revelations about Chris’ infidelity, prosecutors have said his full motive — what possibly could have driven him to such violence — may never be known.

Speaking with ABC, Shanann’s mom said their family had struggled with what happened.

“We did scream at God, yes we did,” Sandra said. “The devil comes to you in any shape, size and form and can be transformed into anything.”

Still, Shanann’s parents “have a strong faith in God and that, along with their son, is what is getting them through each day,” 20/20‘s Robach says. “They are speaking out because they want to set the record straight about Shanann.”

20/20’s episode on the Watts family murders airs Friday (10 p.m. ET) on ABC.