Transcript for Mom Admits to Poisoning Family With Anti-Freeze: Part 3
Reporter: Over the course of 14 months, the quiet, god-fearing staudte family seems as cursed as an old-testament transgressor. Member after member inexplicably struck down. Mark staudte, who fronted a blues band and 26-year-old son Shaun both suddenly dead. 24-year-old Sarah, lying in the icu, her kidneys and brain failing. Now police want to know, is this a matter of bad luck or bad intentions? Which is why Diane staudte, wife, mom and church organist is sitting in the interrogation hot seat. With cameras rolling, detective Mcamis asks about her marriage, and for the first time glimpses a reservoir of resentment. We were still married, but it was not what you'd call a good marriage. Have there ever been any infidelities on either side? He had. He was running around and he would drink and smoke pot. He wasn't a very good guy, is what you're saying? Yeah. Even with all his faults, I still loved him. Reporter: In this police interrogation video, Diane reveals cracks in the foundation of the staudte home. Was there ever any physical abuse towards you? No. Nothing like that? I didn't think it was that bad, but my kids would probably say otherwise. I wasn't happy. Reporter: Now watch as he deploys a classic interrogation technique, shifting from inquisitor to sympathizer. I'm a believer myself, so I understand where you're coming from on that. Reporter: He tells Diane sometimes even the righteous reach a breaking point. So what happened with Sarah? As far as did I do something to her? I didn't do anything to her. I guess I could've taken her to the E.R. Sooner, but I didn't know. Reporter: A major reveal. Diane may have delayed taking her daughter to the hospital. That was just where her story started to crumble. I'm horrible. I'm a horrible mother. Reporter: Then a suggestion that Shaun may have been suicidal. But again, she admits she did nothing to help. He'd been threatening to kill himself. I didn't want to bother because I was tired of it. I'm such a crummy person. Now's your chance to tell us why and to show some remorse, ask for forgiveness. Reporter: Then she drops a bomb. To put it really short and sweet, I knew they were drinking antifreeze. Reporter: Drinking antifreeze? And I was so mad at them, I didn't want to take them in. You knew, Diane, that they were drinking antifreeze because you were giving it to them. I didn't know what else to do. I really didn't. Reporter: And there it is. After two hours in the box, Diane staudte quietly confessing to poisoning her own children with antifreeze. This is a woman who's admitting that she killed her son and tried to kill her own daughter. What are you making of this? I was just totally astonished, to be honest. I couldn't believe what I was hearing. There were times I didn't know what to say or how to react. How long had you been giving them the antifreeze? Maybe a couple of days. And what were you putting it in? Coca-Cola. How much would you put in? Couple of teaspoons, maybe. Reporter: But why antifreeze? She explains that it's easy to get and easy to disguise. The main ingredient, ethylene glycol, a colorless, odorless, sweet-tasting and deadly chemical. Killing someone with antifreeze is a classic way to try to mask what you're doing. Takes time, it's relatively flavorless, and there are no immediate signs after death unless you're looking for it. Reporter: Now it's all pouring out. A full-blown confession. Diane admitting she poisoned her husband mark the exact same way, spiking his Gatorade with antifreeze. Apparently she'd had it with a husband she considered an abusive deadbeat. By then, I hated his guts. He would throw things at me. He would throw things at the kids. And I guess I'd just had enough. Reporter: Killing her husband is awful enough, but a mother poisoning her own children? This mom's motive is downright chilling. Both Shaun and Sarah would just basically, I don't know, trash the house. Never helped support or even contribute. Shaun would be interfering with whatever I would do. So if he was just a constant bother, wouldn't leave you alone. Oh, he was more than a bother. Would a pest, would that be a good word for it? No, it was more than that. Reporter: My husband got on my nerves. I couldn't stand him. And my son, you know, was a pest. I mean, did she seem, like, at her wit's end? No. She, again, throughout, she stayed flat. Even when this -- when she's telling me that information, she's, you know, she seemed unfazed. Reporter: Unfazed, even by the ultimate kicker, why she poisoned her daughter. And then with Sarah, you talked about, you know, she wasn't getting a job and she had these student loans and you were going to end up having to pay for them. That's pretty much. And you just had had it with her as well. I'm not a perpetual killer. I'm just stupid. I regret doing it. I really do. I've screwed up everybody. I've screwed up my whole family. Reporter: But it's too late. Here come the steel bracelets. Right now, you're going to be under arrest. Reporter: Case closed? Not by a long shot. See, police are about to search the staudte home, and will find a purple diary with some very purple prose. This was a key piece of evidence. It will prove that Diane had a partner and eyes on another target in these horror movie homicides.
This transcript has been automatically generated and may not be 100% accurate.