That's a good point about SW coming back from NC. CW was no longer going to be able to pretend his divorce was "final." SW's moving forward with a big gender reveal party the next weekend. He's supposed to be divorced and getting a new place. So he probably felt the self-imposed pressure. Instead of just telling SW, "I AM LEAVING YOU AND WE ARE GETTING DIVORCED."
How stupid that this new relationship with his "girlfriend" of about 2 days was the love of his life and he thought it was going to work out happily ever after. (And that she would never question what happened to his missing family.) He was completely in the infatuation stage - new, fun, sex, nice lingerie, no responsibilities, no daily stresses - all the fun of new relationships. (None of this is a knock on the girlfriend.) And you have your wife leaving you homemade meals in the fridge while you get to go to fancy restaurants and plan secluded resort trips with your girlfriend. It's not real life.
Still... plenty of married guys get caught up in affairs and young girlfriends without ever resorting to this kind of violence and criminal behavior. I don't think we will ever get answers we understand.