The main theme in Mockingjay is that things are not always what they seem.
Katniss has pretty much lived her life with the motto of “trust no one” before she even competed in her first Hunger Games. After the rescue at the end of her second, she is even more confused. She has been told that District 13 does not exist, but it is underground. District 12 is the one that does not exist now—it has been destroyed.
When Katniss joins District 13 in their bunker, she is suspicious of the rebels and their leader, Coin. Coin wants her to be the mockingjay, the symbol of the revolution and the rallying cry for the other districts.
And now Coin, with her fistful of precious nukes and her well-oiled machine of a district, finding it’s even harder to groom a Mockingjay than to catch one. (ch 5, p. 59)
Katniss is right to be suspicious. Coin turns out to be ruthless and motivated, and Katniss is concerned with some of her methods. She pulls away from Gale when he develops weapons that target civilians, and when her own sister is killed she has had enough.
Katniss made a deal with Coin that she would be the one to kill Snow, the Capital leader. Her suspicions lead her to kill Coin instead, thus ending the revolution and preventing a new reign of tyranny and abuse that would have resulted with Coin in charge.
Katniss discovered that Coin was not what she seemed, and the rebellion was in trouble. Too many people were caught up in the idea of ending the Capital rule to think about what the consequences will be. In the end, Coin learned that Katniss was not what she seemed either.
Collins, Suzanne (2010-08-24). Mockingjay (The Final Book of The Hunger Games). Scholastic Books. Kindle Edition.