OT!!! Sorry I love everyone's theories, so here is mine and it's purely outlandish so don't think I think this is true. Buuuuuutttt
I just imagined this and it's completely made up in my head (IJITAICMUIMH): My fictional, made-for-TV movie version of what happened...
Where was CW Sat night when he had a sitter? Did AP from work say they broke up in June-ish? I assume he was on a date? What if what if whatif
It seems he falls for people. He has a new love interest, and everyone knows the first few weeks of an intense affair, one where he was living large as SW alluded to in a snarky FB post from NC. One where they were both talking about running away, both ditching their lives...
The NEW AP (NAP) is like Nancy (or Sid) from Sid and Nancy, or Bonnie (or Clyde) from Bonnie and Clyde. A little dangerous, maybe unhinged. S/he talks about both killing their families. CW is like you're crazy BUT I LOVE YOU because you're crazy and dangerous and new.
S/he visits the home. CW is like hey don't come to the home. NAP is like but we are running away! Let's kill the kids now and dip. CW says let's put the kids to bed and have weird murder fantasy sex instead of actually killing them. SW is coming home soon.
They ... CW hops in the shower immediately after. NAP goes and kills the kids! He gets out of the shower and sees them, murders NAP (who we don't know about, and is probably also in a shallow grave somewhere).
CW rips the sheets off the bed and freaks out, how the hell is he going to explain two dead kids and a mist/ress? He lies in wait for her to come home, and grabs her before she knows what hit her. And has to deal with 4 bodies.
No one ever finds out about NAP, until 20 years later on a Forensic Files episode, season 203, episode 94, they crack the case after an oil worker finds NAP's remains, from fibers from the same bedsheets and CW's DNA.
The End