Predictability v. Expectations

So I read this book recently where the killer turns out to be . . . well, the killer. What I mean by that is the main character is convinced that the person who was convicted of killing her loved one is innocent and that the real killer is still out there. So the book follows her along this odyssey--her fight to bring the "real" killer to justice but in the end, it turns out that they had the right guy all along. I thought this was brilliant although perhaps it just spoke to the part of me who knows what it feels like to have lost a loved one and want answers that are never, ever going to satisfy. At any rate, many other readers were extremely disappointed and angry. I, however, thought it was a really novel way to end a suspense/crime novel. I mean I don't think I've ever read one where the killer turns out to be the killer they had behind bars the whole time! So to me that was satisfactorily unpredictable. (I'm not going to name the book cause I don't want to spoil it for other readers.)

I see this whole complaint about books being predictable all the time. ALL THE TIME. I read reviews of books before I buy them so I read a lot of reviews.  I read my own reviews too. Readers want books that are unpredictable but in the back of my mind, I'm thinking: "Do we? Do we really?"

Most readers I know read primarily in the same genre or genres--there are usually one or two genres they prefer over all others. Many readers I know only read romance or only read crime fiction and so on. Like me, I really only read crime fiction. I'll pick up books in other genres from time to time but for the most part, I enjoy crime fiction the most, so that's what I read. So having read a ton of crime fiction books, I have certain expectations about what's going to happen. There are certain conventions that each genre uses and as much as readers like to be surprised, we also don't like it when writers stray too far from the accepted conventions. In other words, I think it's a fine line between giving readers the unpredictability we crave and meeting our expectations of the genre.

For example, I read a book many years ago that was part of a series where, at the end, the author killed off one of the two main characters. Yeah, killed him. Dead. Gone. Forever. (It would be like watching the Lethal Weapon movies and at the end of the third one, Riggs gets killed off. I mean, really?) So was the book I'm talking about unpredictable? Hell, yes. But not the kind of unpredictability I find acceptable as a reader. I did return to that series but it wasn't for a long, long time.

So my question is: how much unpredictability do you really like as a reader? I mean is it okay if, at the end of a romance novel, the two main characters don't end up together?  Is it okay if, at the end of a crime novel, the mystery is not solved or the bad guy gets away? Or to a lesser degree, take movies for example--would it be okay to have the story in an action movie wrap up without a big, long, protracted chase scene at the end? Or a big old showdown between the good and bad guys? Would it be okay for a Disney princess movie to end without an impending wedding? What truly is an acceptable level of unpredictability in storytelling?