1) I think the reason Midnight Sparkle turned evil while Daydream Shimmer turned good, is that each had different reactions or "effects" of friendship. Once the magic transformed Midnight Sparkle, since she didn't understand the meaning of friendship and it was the remaining magic from the elements of harmony in Equestria brought by Twilight. As said in the first movie "But it is a magic you cannot wield" because Evil Sunset didn't have the true essence of friendship. Just like Midnight Sparkle. Sunset was able to wield it because she knew and carried the traits of all 5 elements, including magic.
2) There have been worst human beings in the world, plus she was willing to do anything to make sure Crystal Prep won. Her reputation was on line, and what if taken from real human beings, reputation is sometimes the most common motivator of despicable acts. Or she just was a poorly written character.