I am conflicted, between CW having planned the entire event, or something that happened and spiraled out of control.
Because the disposal of bodies was so disorganized, I am leaning towards CW went crazy on Cece, frustrated, financial issues, hadn't had to deal with kids for several weeks, then had to take of them alone, no doubt angry that SW left for a "vacation", and he lost it. Then, he "had" to kill Bella, she probably saw everything. Then, SW comes home, just "checks" on the kids, goes to bed, and he kills her in bed. He develops a story, dumps the bodies, and thinks he will have a day or two for clean up.
Or he was so enraged, he planned it all. But, if he did, burying the bodies in oil? And shallow grave? At his workplace. Idiot.