How to hide the murderer in a mystery according to Shannon OCork in How to Write Mysteries part of the Genre Writing Series by The Writer's Digest.
First time mystery writers tend to get wrapped up in their murderer. They can make the murderer more vivid than the hero and draw the attention to him when the writer really wants to draw the attention away from the murderer so the reader is looking in the wrong direction as well as the hero.
*Concentrate your novel on the apparent plot. After all, that's your story. Make the most intriguing character your hero or the victim or the one who might be the next victim, not the murderer.
If you want to see a master at plotting dissect Agatha Christie's stories. In The Murder of Roger Ackroyd it turns out the narrator is the murderer.She used a character trait in the murderer to hide him in plain sight. He(Dr. Sheppard, the narrator) prided himself on never lying. This is the slight of hand that slips the information the reader needs to discover the murderer right in front of them, yet they don't see it.
*You too can hide your murderer through his characteristics and hide your tricks in characters' flaws or foibles.
*If you show a little imperfection in all your characters, you hide the imperfection you need for your murderer in plain sight. If you can show the murderer's imperfection sympathetically,so much the better. No one wants to think the worst of someone he likes.
When writing a mystery:
-don't concentrate on your murderer
-concentrate on your hero
-concentrate on the mystery to be solved and not the solution
-concentrate on the problems the hero faces to work toward the solution of the mystery
-concentrate on the victim. Why him or her? What, of all the thing he was and did, brought him to such an end?