This is one question that has puzzled me. I write novels with a body being found and normally I let the reader know who killed the victim.
Columbo knows who his killer is and so do the viewers. All he has to do is find the proof.
Val McDermid let the reader know who the killer was in ‘The Retribution’ and the story is built up into a peak.
But most novels don’t let the reader know the killer’s identity. There are passages where you get a glimpse of the killer’s thoughts, so you know there is going to be another killing.
Detective Wallander and his team don’t seek to prove that a certain criminal is guilty. They set out to find the killer who has done this terrible crime.
I guess there aren’t any rules. If the story entertains and keeps the reader reading until the end of the story and he or she is satisfied that it has been a ‘good read’, that’s all we can hope for.