reporter: 52-year old scott purk is standing trial on charges that he killed margaret anne purk. in november of 2015, scott purk was found guilty of margaret's murder and tampering with evidence. he won't be eligible for parole until he's 87 years old. when the jury came back with their conviction, it was just like -- what a sendoff gift for my retirement. narrator: one question nags detectives and margaret's family -- why did scott do it? theories abound, and scott is staying silent. my personal opinion is i don't think scott was ready to be a father. i don't know if there's anybody that could ever figure him out, no matter, you know, how much time they have to study him. narrator: perhaps the oddest part of a very odd case is that if scott had just kept his mouth shut, he probably never would have been caught. scott would have gotten away with what he had done had he not said what he did the day of his fire. you know, it was a challenge. he was throwing it out there, letting us know how smart he was. walsh: scott purk got what he deserved, even though it was 30 years later. he

♪ ♪ in the spring of 1985, margaret purk and her husband, scott, were planning a bright future. was pregnant with their first child and was thrilled about becoming a mother. margaret had written to her grandparents letting them know that everything was really well with her, everything was fine, she was happy, looking forward to the birth of their first child. margaret: "i feel great and full of energy. just think, any day now, i'll be a mother. a mother! it's even hard for me to believe but i'm looking forward to it and so it scott. metcalf: for the baby coming, she was ecstatic. me being the little brother and still not very mature, it got on my nerves a little bit, just hearing about it all the time. narrator: margaret's family and friends were relieved by her newfound optimism. she hadn't always been so full of hope. mifflin: there was one incident in margaret's past where, you know, she was upset about something, and she had threatened to harm herself by using a cord from a mini-blind in one of the rooms of the house. it's more or less a feeble attempt because, you know, it'