I don't think it was "turning evil" so much as a shift in his values, strongly impacted by his father's shooting and Appolonia's murder. At the wedding, he seemed to reject his father ("That's my family, Kay--that's not me"). But when he was wooing Kay after he returned from Sicily, his father had become "just like any other man with responsibility for others") including senators and governors who have men killed. I think he must have had an epiphany: that virtue and right can be swept away by force, and that the only way to live was to be a force instead of one who was swept away. He had adopted Vito's attitude ("...and I refused to be a puppett...").