He bombed the courtyard where Prim was taking care of the wounded in order to ensure that all of the remaining Capitol supporters were dead. He did not know that Prim was in the courtyard trying to save lives. Coin couldn't control Katniss. She orders Prim into the bomb zone to take care of the wounded. Knowing she will be killed. She has to break Kat's will and mind. Killing Prim does this. Kat finds out it was coin who ordered Prim into the danger. She kills coin with an arrow in the end while Snow laughs. The then drops dead from all the poisons he's been taking.
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President Snow ordered the bomb that killed Prim.
Katniss's mother and sister Prim heal the wounds
*SPOILER ALERT* He does not kill her directly. He is the one who created the bomb design of the bomb that killed her. Katniss dislikes him now for this.
Ygo Gales Galama died in 1492.
she is katniss' sister and in the last book she gets killed by a bomb that gets dropped by gale ( who thought prim was safe at home not in the square where his enemies were) katniss's best freind