Transcript for Family of woman killed in 'cold medicine murder' haunted by regret
Reporter: They looked like the picture of marital bliss. Newlyweds married for barely 10 months. Matthew Phelps, 28, once attended bible college. His wife, Lauren, 29, was a youth leader at church. She loved kids, she loved helping others. Reporter: But that image of what seemed from the outside like the perfect all-American couple shattered after this dramatic middle of the night call to 911. I have blood all over me and there's a bloody knife on the bed. And I think I did it. Reporter: September of 2017, Lauren was brutally murdered by her husband Matthew. He tortured her. He absolutely tortured her. Reporter: But in a twist that garnered national attention, Matthew blamed the murder on cough medicine. What medicine did you take? I took corcidin. He blamed it on cough medicine, did you ever buy that? Never. You think it was premeditated. It absolutely was. He planned the whole thing from the get-go, in my opinion. Reporter: Earlier this month Matthew pled guilty. His appearance startlingly different. Did you, in fact, murder your wife by stabbing her multiple times? Yes, sir. Reporter: Now for the first time, Lauren's family speaking out about their daughter, their sister, and the importance of having an open dialogue about domestic violence. I feel guilty that I did not talk about domestic violence to my daughters. Before they were married. Because I -- I didn't think that that was something that they would have to ever worry about. What was the last thing you said to her? "I love you." We had a bond like nobody has ever probably seen sisters have. Reporter: Lauren's sister Beth says not long into the marriage there were signs of trouble. She was getting stressed with money. He had just quit a job before they got married, and he still hadn't found a job. She was telling me he was acting different. He always wanted to make Lauren happy, then he didn't care to make her happy anymore. Was she planning on leaving him? I believe she was. He was being very depressed-acting and talking of suicide. And we were afraid for him. We never thought that Lauren was in any danger at all. Reporter: Investigators later confirmed that Matthew had a dark side. Court documents show Matthew was obsessed with the movie "American psycho." And that his private Instagram account shows him dressed as the main character, a serial killer. Also according to court documents, a friend told police that Phelps expressed interest in what it would be like to kill someone. Lauren's family says they had no idea about his dark social media trail. They shared these photos with us from her last days. She looks happy, but her father says he saw otherwise. The last time I see my daughter, it didn't look like Lauren at all. She looked a mess. But -- I regret it now that I didn't ask questions. How do you reconcile that regret, not asking her why, what was going on? I can't. I can't. I mean -- I think about it every day, what I did wrong. What I missed. Reporter: Two days later, Lauren was brutally murdered. Her mother recalls the police knocking on her door in the middle of the night. And the looks on their faces. I can't get that out of my head. It was devastating. And I thought, did Matt kill her? And they asked me, why would you ask that? I said, I don't know, I just know. I just know. Then they told us that he had killed her. And that he had stabbed her. And he had stabbed her so many times that they would have to count the stab wounds in the autopsy. Reporter: Matthew had stabbed Lauren 123 times. The police called him a monster. And we're talking about homicide detectives who see homicides every day. Reporter: In the immediate aftermath of the murder, Matthew attempted to blame his mental state on cough medicine. I took more medicine than I should have. What medicine did you take? I took coricidin cough and cold, because I know it can make you feel good. So a lot of times I can't sleep at night. Reporter: But that argument never held up. And Matthew was sentenced to life in prison without parole. I am sorry that I took away Lauren's life. A life that was deeply connected to so many people. Will you ever be able to forgive him? No. Never. I'll take it to my grave. I'll still hate him. There's no way. No man could do that to any human being. Is there guilt that you didn't and couldn't do enough to save her? I feel guilty that I told her that these are all first-year marriage problems. It angers me that I feel guilt, because it's 100% not my fault. No. But she's my little sister and I'm supposed to protect her. Reporter: According to the national coalition against domestic violence, 1 in 3 women and 1 in 4 men have been victims of some form of physical violence by an intimate partner within their lifetime. I even struggle with calling Lauren's case domestic violence. It feels almost like serial killerish with all his fan taken places. But it is domestic violence and I think that's the thing I want people to know. No two stories are the same. Lauren had a gut instinct that something was wrong and the only thing I wish would be different is she would have trusted her gut to say, there's something wrong, something has to change. This was her freshman year in high school. This was her eighth grade year. This was her favorite dress. She loved this dress and wore it until it could not fit. What would you tell Lauren right now? I would tell her that I hope that we've made you proud. And that she will always be present in our home. What would you tell your daughter? I'm sorry I let you down. I think that all the time. I failed as a father. I should have seen it. But I'm not going to let the rest of them down, not no more. Reporter: For "Nightline," I'm Paula Faris in north
This transcript has been automatically generated and may not be 100% accurate.