No, not that.
The McCanns left their children alone that evening, showing a definite disregard for their well-being, and a willingness to put their personal goals or needs ahead of their children.
There are dozens of foreseeable accidents that can happen to children that age left alone--with or without an unlocked door.
If they could put their needs ahead of a living child, they most certainly could put their needs ahead of one that was dead.
Again, I don't think the McCanns deliberately killed their child. As I see it, three possibilities:
1) Maddie was abducted by a kidnapper who deliberately chose her over easier targets, and within a very small window of time
2) Maddie wandered off via an unlocked door, and then was either abducted (a stretch) or injured and hidden/buried somehow
3) Maddie suffered some kind of injury resulting in death, including something related to a medicinal dose, and rather than lose their licenses/livelihood and remaining children, and knowing they are well-meaning and caring parents overall, her parents chose to cover up her death.
Occam's Razor says the most likely/simplest explanation works. Which one of these is the most likely then?
I don't want to believe the parents did #3, but I will point out that most of their behavior after the fact backs up #3.