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 h