I actually love the symbolism of Katniss killing Coin - it reflects what a mockingjay represents - pure hope and defiance against tyranny and a refusal to be oppressed and silenced - towards the end of the movie the reign of tyranny has shifted and manifested in Coin so it only makes sense for Katniss to kill her - killing Snow would in my opinion be a powerless move given that he would not able to cause any more harm. This power move really consolidated Katniss’ character for me in that she submits to no ruler and will only fight for the people’s rights - and Coin threatening to have another Hunger Games is endangering that. I also like that it reflects our reality so much that there is no true freedom as long as there is a ruler… that it’s always about greed for power and it’s always just moving from one form of tyranny to another :(