who was this matt kidder, whom the nelsons loved like their own? interrogation room matt opens up to police revealing he feels guilty he didn't stop the killer. >> i feel like i was there and i could have done something if i drove by that night or if i had known. >> when "dateline" continues. feel stuck with credit card debt? move to sofi and feel what it's like to get your money right. ♪ move your high-interest debt to a sofi personal loan. you could save with low rates and no fees. earn $10 just for viewing your rate and get your money right. ♪ >> reporter: a murder investigation is often a collection of stories. whodunit stories, yes, but also love, loss, regret. john mcdowell was innocent. but he didn't feel innocent. >> the only thing that went through my head is that it really, it was all my fault, because if i would have never messed up and if we would have never split up, then i would have been there to protect her. >> reporter: i think you still feel that way a little bit, don't you? >> yes, sir. >> reporter: does she intrude upon your thou

like matt kidder. >> when i got the call, i walk out of the room to make sure the sky wasn't on fire.essica, wouldn't be dead. >> when detective davis interviewed matt, he encountered a man still apparently grieving. >> she had a really good heart. >> yeah? >> good mother. good friend. she -- i don't think i've ever heard her not put herself out for somebody in need. >> as so davis was sympathetic. and matt opened up. talked about his brother really teasing relationship with jessica. >> me and just, we always had the relationship where we have inside sexual innuendo jokes. even though we have never had interest in each other like. that i've kind of always enjoyed -- that being able to do around would somebody without, you know, it's never going to lead anywhere, so it's like everything is on the table. >> and like brother and sister, he said, they sometimes get physical in a playful way. >> we'd fool around a little bit, but i, mean -- >> do you mean? >> it's just kind of like she would pinch me, i would put her in her side. just like teasing. sibling teasing, i guess you could call.