In the Grimm tale, why did the evil fairy cast the curse on the princess? Because she was not invited to the party the king and queen threw when the princess was born. While some people might find this offensive, I never did feel it warranted a death curse. However, in Maleficent, the story changed. They gave Maleficent a reason to be mad at the king and a good one at that. Though I was not a big fan of the whole "woman scorned angle", I did feel it was a better reason than just not being asked to a party.
What I liked best, though, is that true love's kiss didn't come from the prince. Disney has been changing this concept for a while now. In the Once Upon a Time TV series, it was a mother's love for her son. In Frozen, it was sisterly love. They've begun to expand on this concept and I like that. There is so much more to love than between a man and woman and I like that Disney has been exploring that.