Not sure if this theory has ever been posted, but it may be that Karr (or another intruder) did kill JonBenet, after which the parents composed the ransom note together in a blind panic.
Upon seeing their daughter's bashed, garotted and obviously defiled dead body, they knew that the finger of suspicion would be pointed their way and that's when the ransom note idea came into play.
Maybe that's all the Ramseys are guilty of - giving false information to law enforcement over the true authorship of that note. Maybe that's the only secret they have been keeping all these years.
It doesn't make them killers, just naiive and stubborn. But who on this forum can say they would behave in a more composed and rational way after finding a loved one bashed and murdered in their own home in the early hours of the morning?
Just my opinion. Thank you.