The rebellion, including Katniss, has attacked the Capitol and are nearing on the presidential palace where Snow is. He's tried to protect himself 由 placing all the Capitol's children between them and him. However, the rebels drop bombs on them anyway (Gale helped designed these bombs which is why some 粉丝 blame him a bit). Prim is 由 now working with the rebels' medical department so she rushes in to give medical assistance to the kids. The bombs have a 秒 explosion built in and that detonates. Prim is killed.
**SPOILER ALERT!!** No, Katniss did not kill Prim. In Mockingjay, in the Capitol, all the children were rounded up and formed a child barrier around President Snow's mansion. Gale and Beetee had recently designed a bomb in District 13 that dropped small silver parachutes, (the kind used in The Hunger Games), on the children, which they opened, and exploded. They were designed to explode once 更多 as the nurses rushed in to aid the injured. Prim was one of the nurses, and the bomb exploded while she was inside the exploding range. Therefore Katniss did not kill her, the bomb did. And because Gale helped Beetee 设计 it, he is labeled as killing Prim as well. For this reason, Katniss holds her distance from Gale for a while.