Why did Khrushchev denounce Stalinism?

He didn't denounce Stalin. In fact he was one of the most strident cold war Soviet premiers of his time. He went to the United nations and pounded his fists and shoe on the podium yelling when a they voted against Russia. He put the missiles in Cuba creating the Cuba Missile crisis and a 60 year boycott of Cuba. No, he was a long standing Soviet communist party member and had fought in WW2. He wouldn't have dared to denounce Stalin.