The "Who" in Whodunits Can Be Tricky
My mom used to complain that she’d figured out the identity of the bad guy in books by Mary Higgins Clark way before the big reveal. Honestly, that never bothered me. I’ve always loved Clark’s storytelling ability, and even when I correctly suspected who the villain was, I kept right on turning the pages.
Still, as any mystery writer can tell you, endings for our whodunit tales can be tricky. When it comes to revealing the guilty party, we want to leave our readers satisfied. We’re advised not to be too predictable. Surprise is good. So are unexpected twists. 
But there is a danger. We do have to lay some groundwork. We have to develop plenty of hints and foreshadowing not only for our red herrings, but for the actual culprit, so that the reader feels the ending, while not necessarily expected, is understandable. If we don’t, we risk irritating our readers.
Recently, for example, I read Karen McManus’s Two Can Keep a Secret. McManus is a wonderful YA mystery writer, and I enjoy her work. I was riveted throughout her story—until I got to the end. In her story, teenage girls had gone missing and murdered, and the entire focus of the book had been on the teens and their slightly older siblings who had some connection to the victims. But (spoiler alert!) the revealed killer, a prominent attorney and the seemingly happily remarried stepdad of one of the protagonists, had barely appeared in the story. Unless you count his occasional “flaring nostrils,” there is nothing about his demeanor or reputation to indicate that he could possibly have been involved in not one but three murders of teenage girls he’d romanced in the course of two decades. As a reader, I just didn’t buy it. I felt cheated. It reminded me that we writers need to spend as much time fleshing out our villains as we do our heroes and our red herring characters.
All in all, this mystery writing business is complicated! But I find that I learn so much about what to do and not do as a writer not only from my own plentiful mistakes, but from what works and doesn’t work for me as a reader. It’s a lifelong learning process, and it never fails to engage and surprise me.