Stupidity: Unless Snow has solid proof like pictures or video why should Katniss believe her main enemy's word that it was Alma Coin, not him, who killed those kids?
Suggested correction: This is a question and not a stupidity. Katniss didn't just accept Snow at his word. Earlier Gale had mentioned an attack strategy to inflict maximum casualties on medics that matched the attack on the Capitol. Then Gale could not deny his part in Prim's death. All the evidence pointed to Coin being responsible as Snow suggested.
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