Great book by Bill James (sports writer who happens to be an excellent writer, period) called Popular Crime: Reflections on the Celebration of Violence has an interesting theory. All in all I cannot recommend this book highly enough to a fellow crime devotee.
But James describes a convincing scenario involving an intruder whose main goal was to hurt the father, John Ramsey.
Anything credible I've read points to a person outside the family. Parents like the Ramseys don't suddenly go from normal people to sadistic child murderers or crime concealment experts overnight.
And seriously, if your child, accidentally or not, kills your other child is your best idea really going to be, "I'll hide our dead daughter's naked body, you write a ransom note making this look like a secret group's foreign kidnapping plot!"
Now if James is correct in his theory then killer was nearby/remotely surveying the Ramseys, and/or listening to a police scanner, waiting to collect the ransom.
And by 9:30 a.m. Ramsey had arranged with the bank to get the $118,000. If the police had not been called to the house the murderer may have gotten the money, though money may not have been the primary motivator.
At any rate it is a fascinating case likely to cause eternal debate.
E: Spelling errors
Popular Crime
I loved Tina Fey's Bossypants
I'm currently reading Popular Crime and its a great dive into the phenomenon of the media's obsession with true crime. Also added in are some very interesting stories of older and more obscure crime stories from as far back as the 1700's. Its been a great read so far if you're into 2spooky crime stories like I am :p