That first movie was disappointing. It was cast well. Jolie had some cool scenes toward the beginning. Then they decided they'd make Maleficent the hero rather than the villain. It'd've been more interesting if they just left the story alone, let villains stay villains, and stopped lost the obnoxious rewrite of the ending.
If she stayed good at the end of the first movie, then maybe she isn't the mistress of evil. It will probably something stupid like her mom or something.