I think he promised SW he’d work on their marriage in order to pacify and shut her up, but the clock was running out. Shanann would be coming home for a long spell that night. There would be no more free and unaccounted for hours for sex and games with NK, and NK would very quickly discover that his divorce wasn’t as finalized as he told her it was. The game was over, it was all coming to a head, and he chose NK, and annilihated his family to get her. I don’t think he anticipated NK would betray him by going to the police, or that she wasn’t as infatuated with him as he was with her. Remember, the night before NK texted to her friend, Charlotte Nelson, “Don’t tell nobody. I haven’t made up my mind on him yet. Like he seems too good to be true.” (Discovery page 715) Once he realized NK had turned on him, he realized it was all over, they were never going to be together, he might as well confess and plead guilty because there was too much damning evidence against him, why fight it? He cared so much about his image, if people hated him already, imagine how much they would have hated him when everything he did came out at the trial. He didn’t want that. I won’t be surprised, however, if he does an interview later down the road, saying Shanann killed the girls, he only plead guilty to protect her because he’s really a good guy.
Why didn’t he just move in with her and give full custody to SW? NK would have been fine with it