It’s misdirection. Say, for instance, this was a familial murder. The criminal is thinking “I’ll make it look like she was kidnapped by a stranger and held for ransom, so the police don’t look at me.”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.
And it worked, for a little while. But it won’t work in the long run.