Many people have read series like The Hunger Games, Divergent, and The Maze Runner, all of which include 3 main books with an occasional prequel or side short story. The main thing I tend to hear when people reach the final book is how disappointed they are. It can be the storyline, the fate of certain characters, or the resolution to some of the conflict.
Having read all three series, I can understand why people are reacting. Yes, its heartbreaking to read favorite characters either dying or fighting to save the main character or perhaps a pairing ends up with someone else. But take a step back and think about this: In order to show how high the stakes are in a fictional world or dire situation, people die. Not just the bad guys but some of the neutral figures and beloved allies have to. If they didn’t, it would become a very cliche story and you’d probably be complaining about how the story ended too neatly with a perfect ending.
Also consider that the authors could be under stress from their publishers or fans to meet certain expectations. Some people feel that Mockingjay had a rocky ending that seemed forced but I feel that in order to get to the conclusion she wanted while giving some fans the resolution they wanted, Suzanne likely had to plan out certain deaths to symbolize the message of dying for a good cause while showing that love can persevere in very difficult situations if the bond is there.
I still think back to J.K. Rowling saying how she regretting killing off a minor character. Recently, she’s apologized to killing off other characters in the series via Twitter. You have to admit, she wrote an impressive roster of characters and eventually some of them had to succumb to death in some way in the series. Otherwise, it would be a nightmare for her to manage all of them in the conclusion.
I’m not saying you aren’t allowed to slam your book shut, throw it across the room, and scream your frustration at the ending. But instead of bombarding the author with hate, I think its best to take a deep breath, think about how difficult it likely was to write that scene/character fate/etc., and remember that the author wants to give you a fairly realistic scenario of what can happen when the stakes are high.