The Triforce doesn't change people who take it. It only ever reacts to what's already in their heart. That's kind of the entire reason why A Link to the Past's plot works the way it does. Ganon was already a blackhearted evil bastard when he touched the Triforce and that's what created the Dark World. Basically the same exact thing happened here with Ganondorf, so he had to have been pure evil already when he left to get the Triforce.