Human Twilight's questionable transformation

[Discordrules123 10]

I've only seen clips of "Friendship Games" from reviews but I know the story well enough, and one particular thing I've noticed was Twilight's transformation into Midnight Sparkle, which is both cool looking and at the same time confusing. I can understand somewhat that her appearance changing was because of all of the magic that she was carrying around, but the fact that it made her turn psychotically evil was rather far-fetched. First of all I don't think normal magic would brainwash or change someone's own personality, even if they were infused with a whole lot of it. If anything it would most likely cause them to have a very hard time controlling it. Now if it was dark magic that was infused into Twilight, or if some of it was somehow mixed in with the normal magic that she was collecting, then that would actually make more sense.