He could have left the note, taken the body and buried it? Why didn't he? Because he was on foot and didn't know the area well enough and he had more or less put a time on this horror.
I've always thought, just my opinion, that the ransom note was written before the murder...that the killer was in the basement when the Ramsey's returned from the party.....he could have sat there for hours writing and waiting....then the kidnapping went wrong, somehow, and he murdered her......but I don't think this is the guy.