Here's a breakdown of the key reasons:
* Coin's manipulation and cruelty: Coin is revealed to have been manipulating the rebellion, using propaganda and fear to maintain her power. She even proposes using children as soldiers, mirroring the Capitol's past actions. This echoes the very system Katniss fought against in the first place.
* Coin's resemblance to Snow: Coin's methods and ambitions mirror President Snow's, showing a potential for tyranny in her leadership. Katniss sees her own experiences with Snow's tyranny reflected in Coin's actions, prompting her to act against the repetition of history.
* The Capitol's legacy: Katniss understands that simply overthrowing the Capitol doesn't automatically bring peace and freedom. Coin's actions suggest a new form of tyranny, a continuation of the very oppression the rebellion sought to end.
* Protection of the children: Katniss is deeply protective of children after witnessing the horrors inflicted on them during the war. Coin's proposal to weaponize children is a horrifying prospect that Katniss is unwilling to tolerate.
* Personal agency and defiance: By killing Coin, Katniss takes control of her own destiny and defies the manipulation she has experienced throughout her life. She refuses to become a pawn in someone else's game and acts as a symbol of individual agency against oppressive forces.
It's crucial to understand that Katniss's act isn't about revenge. It's about recognizing the potential for tyranny within any system and breaking the cycle of violence. It's a difficult decision, and one that raises complex ethical questions about revolution, leadership, and the responsibility of power.