I think Lou Smit is correct in his general assumptions that anyone, parent or intruder, would have written the note prior to the killing since the excitement level afterwards would simply be too high to sit down and write anything at all.

Ofcourse one's first question is why leave a note at all? The next question is why leave a handwritten note in this day and age of a wealth of alternative sources of untraceable materials. Then ofcourse why leave such a lengthy and absurdly fanciful note?

Some suggestions have been:
Safer to write a note there than to be caught with a prepared text in his pocket if there is a burglar alarm that he trips.

Leads a transient lifestyle and has no alternative writing materials or place to put them.

Wrote the note to while away the time awaiting the family's return and had no prior intent to write that note or even any note necessarily, simply a whimsical flight of fancy.

Wrote the note solely to cause additional worry the next day for his 'entertainment'.

Some have seen two authors, one male one female. Some have seen intent to disguise handwriting, some have seen no worry about handwriting because he is nowhere near the group of people ever likely to be asked to submit writing samples.

I certainly see no reason to leave a note at all, much less such a long and weird one. Parents if they chose to concoct a kidnapping scenario would have written a simple 'we got your kid, get the money together and wait by the phone, no cops or she dies; they would not have written about foreign factions and brains and tactics and countermeasures.