In Catching Fire, Katniss is in District 13 now. Why is she in district 13? Why did the villians bomb the last district to smithereens? That's so mean to do isn't it?

To bomb a district to smithereens means that the government is punishing the people of the district for whatever "sins" they have committed against the government. Katniss has come from District 13 originally and now lives in her beautiful house provided because she won the Hunger Games.