Yet another problem people had with the Mockingjay book is that we've had all this build up toward killing Snow and having Katniss do it, and we never get that payoff. Katniss kills Coin instead. It's obvious why she needed to kill Coin on a plot level, but it was important it happened on a thematic level also.
Like I said in my earlier post, The Hunger Games has never been a good guy vs. bad guy story--just look at the first installment. Katniss isn't pitted against other villains. She's pitted against other youth who are also victims of their society. Katniss's real enemy is the wicked appetites of human nature, the natural man. The Hunger Games has always been a story about the good of human nature and even more so, the wickedness of it.
Also like I mentioned earlier, just as Katniss becomes the face of the rebellion, Snow becomes the face of the evil:
[Snow] embodies everything in the books that's evil, especially as the series continues. I find it interesting how much he gets made into THE villain, when in reality, he's just the face of the wickedness of humankind, like how the Mockingjay is the face for the rebellion. After all, he wasn't the one who started The Hunger Games. He wasn't the one who established the Panem government. He's evil, yes, but more than that, he represents evil.
As the series goes on, we become very focused on Snow. But remember who the real enemy is. It's not Snow himself. It's the evil side, the bloodthirsty side of the human condition.
As I was watching Mockingjay again over the weekend (yes, I'm one of those people), it hit me that by Katniss killing Coin instead of Snow, it shows again that Katiniss's enemy isn't with a person, it's with the natural man. That's why it was important to show that Coin was as bad as Snow--because the enemy isn't a person, it's a nature. Of course Snow isn't the only one who believes in having The Hunger Games. If he was really the only person, then Panem wouldn't be set up the way it is. The Hunger Games wouldn't have been able to exist, because there wouldn't have been an audience, a whole organization, for it. The truth is, all humans have a dark appetite of the flesh (but it's our choices that develop or hinder that appetite). As it was from book one, Katniss's battle is against that part of the human condition, which Coin isn't immune to.
By Katniss killing Coin, she's symbolically killing that human condition, perhaps much more so than if she had just killed Snow. We'd become so focused on Snow, and even the characters seemed to think he was the be-all end-all of wickedness, but Collins shows that it's bigger than him. It's human nature.
Can we take a moment to appreciate how brilliant Katniss was to take that opportunity to kill Coin? Snow was sentenced to be executed, so of course he was going to die, but she wouldn't get the same kind of opportunity to kill Coin again.
While Katniss killing the human condition comes with a high cost (her sister), we still see that although Prim, who represented everything that was good in the human condition, died, goodness and innocence and purity is able to be reborn (figuratively and literally).
In the epilogue, Katniss remarks that the way she gets through the horrors she is haunted with is by making a list of every good thing she's seen someone do--the good side of human nature. It's the good that helps her cope. It's not immediate, but in the grand scheme of things and the worldly truths of the story, in the very end, the good side of the human condition does triumph, even if the person who represented all that was good died in the process. (See, this story doesn't end as depressing as everyone makes it sound. It's just that a better future comes with a very high cost, and leaves painful scars.)
I especially like in the movie version (it's slightly different in the book version), how one of the first scenes is of Katniss discussing she'll never have kids, implying it's because of the evil society she lives in, and the last scene shows her with children, who are innocent, pure, and everything good. It testifies of a better world, one where she doesn't have to worry about them starving or dying. One where the good nature of humankind is stronger.
Don't forget to enter my Comic Con Giveaway, where one of the prizes the winner can pick from is a Hunger Games shirt.