In watching CW's interview again, I get the feeling CW HATED SW. Absolutely hated her, and that the MLM business was a huge annoyance to him. When he says that it's normal for SW to ignore phone calls from him but not "her people", I feel like that bothered him quite a bit.
I also notice, when he speaks of doing things for the girls like telling them what not to eat, or seeing them watch Bubble Guppies, he says "I miss that". It hasn't even been 24 hours at this point, but he talks as if a substantial amount of time has passed since the girls disappeared. IMO, that suggests he knew he was going to kill them for days, even weeks before he actually did, and they were already dead in his eyes.
Also, he never shut down the interview. He waited for the next question until the reporter was done. No urgency to wrap it up, to check his phone or to hop in his car and start searching. The interview almost seemed like a "test run" to see if he could get through it without cracking.