Yesterday, I read a book (which will remain nameless) that made me want to bury it in the sand. The characterization was deep, the writing was vivid, and the villain was such a minor player that by the time he was revealed I felt betrayed. In psychological and domestic thrillers/mysteries (the genres in which I write), the villain should be hiding in plain sight. Don’t tell me the butler that showed up every now and again to deliver a cup of tea is the kidnapper–especially not after making me suspect the victim’s mom. It will feel like the bad guy came out of nowhere and that the writer manipulated the reader’s emotions rather than actually created a puzzle able to be solved.
In my opinion, the best mystery writers make the villain a POV character or close to it. He or she should be someone in many of the scenes, ideally someone even trying to help with the investigation. We should have a sense that we know who he or she is and what his or her motivations are. It should feel like we had a shot at figuring out that the person was, at least, hiding something. 
