For the serial killer/stranger contingent... what kind of serial killer just kills with no sexual or ritual in the crime, then locks the door? A serial killer wants the world to know what they did, not hide it behind a locked door. That is their masterwork. They are proud of it. They want it to be discovered.
If you want to delay the discovery of the bodies what kind of serial killer locks the doors but leaves two people in the house alive? You think an actual experienced killer thinks locking the door will actually buy time as why wouldn't they think someone has the key so why bother? How could they know that?
This is a crime of familiarity. Locking the door was consciousness of guilt. This will not be a stranger nor a serial killer. Look to the local motivation for this murder just like the Ramsey case. I fear the bumbling police fouled the crime scene, bungled the interviews, probably assumed it was a serial killer right off the bat, but then when the evidence pointed back to the house, just like the Ramsey case, they puckered up and decided to keep everything under tight wraps when getting information out there is what actually could solve this but they don't want to show to the world how incompetent they are.