Kingdom Hearts Maleficent

Yea, when she was stabbed with the keyblade her full powers emerged causing her to go into a fit of rage or something. So she turned into her most powerful form. When Sora did it yes he turned into a heartless (also a Nobody Roxas) but that is because Maleficent is basically pure evil and the embodiment of darkness so you have to ask yourself does she even have a heart to begin with. Though in part two you could argue she does a few good deeds . . . but basically it's for her gain anyway.

Also it was kind of obvious she was going to come back once you fought with Pete in front of Master Yen Sid's castle. That was like major foreshadowing. As like most Disney characters . . . anything is possible.