How to use a confession in a mystery
Question: Can you use the confession of a thief, for example, as a way to answer some questions that the reader might have?Answer:
Confessions, like interviews with witnesses or suspects, are one way for the sleuth to obtain clues and other information.
You could certainly have someone confess to a minor crime and have that confession contain clues to the major crime.
An example from real life: as a youth, the boxer, Jake Lamotta, committed murder and then confessed to a burglary that happened at the same time on the other side of town to give himself an alibi. He went to the reformatory for the burglary, but that was better than being convicted of murder.
However, as that example shows, the best interviews don't just answer questions. They also raise new questions or introduce lies or red herrings to throw the sleuth off the track.
Also, you want to avoid having the villain simply confess to the main crime for no good reason, because that makes the solution to the mystery too easy. It's best to have the sleuth use his/her intelligence to come up with a fully proved solution at the climax, at which point it doesn't matter what the villain says because the sleuth is obviously right.