If IDI, then it may have just come down to having nowhere to take her. It really could be that simple. Here are a few other possibilities (I’ve posted the following elsewhere, so some may want to skip over):
1. a kidnapper could have intended on murdering and hiding his victim in the house right from the get-go, possibly believing that the Ramseys would not call the police and that he could collect his money before the parents discovered the body (why would they look for it?). Murdering and hiding the body in the house relieves him of the risk of having to handle, transport, hide and return/dispose of his victim and reduces the risk of forensic evidence accruing.
2. a molester who happened to kill (as opposed to a killer who happened to molest) could have created the note as a means of hiding from himself and/or others his perverse desires and true motivation. Wiping, redressing, covering body and elements of a kidnapping (cord, tape, note) all could have been done as a means to misdirect. “We know that offenders are more reluctant to admit sexual motives than other types of motives (e.g., profit, revenge, anger, power). Some offenders may not even realize their true motivation. An offender may eventually request a ridiculously small ransom for a child he had abducted to molest in an apparent attempt to convince others, but primarily himself, that he is not a sex offender”
http://www.missingkids.com/en_US/publications/NC70.pdf
3. a killer wishing to direct suspicion towards the occupants of the house (thus, away from himself)
4. a killer wishing to create an enduring mystery
5. a killer hoping to create for the parents a sense of false hope mingled with hours of angst and pain reaching its peak when the body is discovered
But, really, we don’t know. IMO we need an identified suspect if we hope to realistically answer this question.
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AK