My biggest problem with Mockingjay isn't Prims's death (although the manner it was handled was really awful), but it's that Katniss herself does absolutely nothing the entire book. Until the last act, her purpose is to sit around and occasionally throw out a speech about friendship and freedom or whatever while the people around her do things that are far more interesting but are never elaborated upon.
Then when you do get to the final act where she actually starts to DO something, whoops, turns out it was all a pointless distraction so that more characters could get killed off and she could see Prims die.
Katniss as a character became spectacularly bad after THG as well.
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