Amateur novelist; a few things to consider:
In an RDI scenario the ransom note would make perfect sense if not for the body being in the house. The body in the house contradicts the ransom note. Oddly, this contradiction is a reason many RDI (as Ive known and understand them) commonly cite for their belief: no kidnapper would leave both note and body.
However, this contradiction actually goes towards disproving RDI, as it is not reasonable to fake a kidnapping, and then call the police before disposing of the body (it is reasonable to create self-incriminating evidence and give it to the police; it is not reasonable to create a scene indicative of an inside job when youre trying to direct suspicion away... ).
As to why an intruder would leave both note and body, several possibilities present themselves; examples:
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. <quote> 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 <unquote> <1>
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
6. Virtually any reason you can think of for a Ramsey to write the note with the body in the house works just as well for an intruder; the possibilities are endless and it is a blatant error of reason to say otherwise. The claim that there was no purpose whatsoever for an intruder to leave a phony (or real) ransom note is false.
<1> Child Molesters Who Abduct: Summary of the Case in Point Series Edited by Kenneth V. Lanning and Ann Wolbert Burgess
http://www.missingkids.com/en_US/publications/NC70.pdf
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AK