N1 panda. Not sure exactly why she did it, but for whatever reason she thinks that she has the right to determine the president's fate and that of the entire world for herself.
Well, if I remember right, The Hunger Games that Katniss voted for is just for the citizens of the Capital, not the districts. The previous Hunger Games were only for the districts for uprising against the capital in the previous war where District 13 got blown up. The Capital was free from those Hunger Games. So Katniss was probably thinking that this is revenge against the Citizens of the Capital, therefor making no change to the madness that existed.
It's been a long time since I read the books but this is what I think...
It's kind of a representation of how rebellions almost never work. Take the Russian Revolution for example... Once the leaders of the USSR were overthrown and communism kicked in, things seemed as though they would go well but in the end, the people of Russia suffered worse conditions then they had previously. Just as the districts thought the hunger games were over and they could finally live without the capitals control, they only changed who was in charge.