In light of the fact that this man frequented reddit and similar sites, I wonder if this murder was a recapitulation of the story "Autopilot" found on the page /r/nosleep. In this fictional tale, a parent makes a misstep in their routine, leading to them forgetting to drop their child at daycare. They went through the entire day without noticing that the child was in the car, despite going in and out during the day and only realized their fatal mistake when they discovered that the child was not at daycare later that day. Perhaps the couple, driven by a malicious want to be child-free, read this story and, after researching its likelihood to work, executed it. The couple may have thought that staging their child's death in this way would lead to it being ruled as an accidental death, leaving them to get off scot-free.