I mean this as a serious question because I'm very confused about all the comments that suggest the kidnapping and/or ransom demands are just a ruse to cover for the murder. I don't really understand what that means. How would further entangling yourself with a crime be helpful? It changes nothing.
The theory being discussed is called crime scene staging.
This is the deliberate alteration of a crime scene by an offender before police arrive to mislead investigators, or redirect the investigation. It can also be done while the police are at the crime scene and afterwards. It is the cover up of a crime. A hoax for the purpose of avoiding arrest.
This can be done by altering, changing, deleting evidence. And by providing false information in an attempt to misdirect, confuse and influence the outcome of the investigation, to avoid arrest. This can be done by providing verbal misinformation and/or written material such as suicide or ransom notes.
For example a domestic murder may be disguised as a kidnapping. The offender and sometimes the family will hide the body, make it look like an intruder entered the home. This serves to get control of the crime scene and LE. Police will immediately scatter outside the home looking for the kidnapper and victim. The killer has time to leave the jurisdiction or dispose of more evidence.
Sometimes police suffer from threshold bias where they believe the staging efforts and discard all evidence to the contrary. It can also serve to confuse the jury if the case goes to trial.
Douglas, John E. Et al. (1992)
Crime Classification Manual Lexington Books
Geberth, V.J. (1996)
The Staged Crime Scene. LAW and ORDER Magazine, Vol. 44, No. 2, February. (Former Commander, Bronx Homicide, NYPD)
Pettler, Laura, PhD (2015)
Crime Scene Staging Dynamics in Homicide Cases 1st Edition
Ramsland, Katherine PhD (2016)
Staging Murder. Psychology Today, Law and Crime.