I was shocked that the sweetest filly-sitter ever, who supposedly spread joy wherever she went and had been such an important part of Twilight's life, could possibly change SO MUCH. X_X However, I did quickly suspect that something was up...I even remembered a tiny "Changeling" spoiler I'd come across online before watching the ep, which I looked away from! So at some point I put two and two together, and realized that there was more to the situation than met the eye. ;)
She's evil because she is a changelings and in the episode she threw away AJ's food, said to pinkie pie that she's to baby talking (Party for a 6 year old), tell Rarity to change dresses un politely, Not being nice to anyone for eg: Twilight. Made Shining armor fall under her control and portend to be Me!
She was very mean and cruel. She told Rarity to change the dress in a mean way,she said that Pinkie Pie makes parties for babies,she threw away Applejack's pastries,and she also told Twilight Sparkle,"Am I interrupting anything?" In a mean way.