Rule #01 — If the obvious suspect is guilty…
You're probably only halfway through the story.
There is a particular kind of relief a reader feels when the obvious suspect turns out to have done it. It is also, usually, a warning. Not that the book has failed, but that it is not finished with you.
The obvious suspect exists to absorb attention. They have the temper, the debt, the public argument, the missing hour. They are assembled out of everything an investigation is trained to notice. That is exactly what makes them useful — to the plot, and often to whoever is actually responsible. A story that hands them over early is rarely confessing. It is clearing the field.
The interesting question is never whether the obvious suspect is guilty. It is what their guilt is doing. Guilt can be borrowed. It can be arranged. A person can be entirely responsible for one thing and entirely innocent of the thing everyone is looking at, and the distance between those two facts is where most of a thriller actually lives.
So when the case closes at the halfway mark, the right instinct is not suspicion of the writer. It is suspicion of the room. Who benefits from the answer arriving this cleanly? Who stopped being watched the moment it did?
The obvious suspect is not the end of the investigation. They are the first thing the investigation was allowed to find.