Because Peeta is good. He’s all that’s right, he’s everything The Capitol isn’t. The Capitol government is about oppression, about the Games, doing nothing to help anyone but themselves and the citizens of the Capitol. They’re about everything that is wrong and corrupt.
But Peeta. Peeta Mellark is the opposite. He’s right, and pure, and innocent. He’s what the rebellion stands for; Katniss is the Mockingjay but Peeta was on fire too. He kept her alive the first few days in the Arena, leading the Careers away from her, convincing them to just sleep instead of killing her on the spot when they very well could have, because he trusts that she’ll be able to get her way out of it. He pleads with her not to go get the medicine that will save his life, because he doesn’t want her to die.
In an every-man-for-himself place like the Hunger Games, Peeta is as self-sacrificing as one could be. He would gladly die for Katniss. And the Capitol doesn’t like people who would die for the sake of others. That’s the core of rebellion.
The one thing that Peeta says is that he doesn’t want the Games to change him. He wants to be himself until the very end. While everyone else is focusing on how to best kill others, Peeta is focusing on how to best keep his sense of self. This is an enormous idea for a teenager.
In Mockingjay, he is torn from himself. He’s someone else, a killer created by the Capitol, an evil being that is everything that he is not. While he is good, and just, and loyal, and kind, his new self is the opposite. This is the last straw for me, the last of the last straws that is the entire trilogy. Because Peeta was the goodness that centered everyone, and now that he is good no more, the Capitol has simply gone too far.
Peeta is too pure to be pushed into the most unpure of things - the Hunger Games. He contrasts the entire idea of the thing just by his being. He’s the reason the series means so much to me. Not Katniss, not Gale, not Prim, not Rue. It’s Peeta. Because while none of them deserve what happens to them, Peeta is the one whose personality yells at you, SCREAMS at you that THIS IS NOT FAIR.