One of the most powerful lessons from Mockingjay revolves around the novel's criticism of government, corruption, and power.
"Absolute power corrupts absolutely." (Lord Acton)
Katniss feels this keenly throughout the novel as her mistrust of Coin grows. She creates her own agenda when she decides to go after Snow inside the Capitol. After witnessing her sister's murder outside the palace, Katniss realizes that she has been used once again. In the ultimate irony, it is actually President Snow (the poster-child for government corruption and manipulation) who makes Katniss aware that Coin has in fact played her as an expendable pawn.
Although Katniss takes this lesson the hard way, still reeling from the shock of Prim's death, she learns from her mistakes, and when given the chance, seizes the opportunity to assasinate Coin. Katniss could not risk letting another ruthless politician become elected--in short, allowing another would-be 'Snow' to come to power.