As soon as she gets to be a princess, wants revenge on her stepfamily but Charming refuses to participate. So, she kills Charming and sweetly takes over because nothing will stop her in her plans of revenge.

Also Snow White and Cinderella. They are truly evil.

Aurora. Goes and kills everyone else, including the other princesses, and crowns herself the new Mistress of Evil.

"SUPPER!"
*Dwarfs run in and eat* *Dwarfs die*
Snow White laughs evilly then ambushes the castle with an army of evil forest animals*
With a smile and a sooooooong
Okay, that's a little far-fetched.

The reason the stepmother hated her all along was because she was a spoiled brat who was always making fun of her stepsisters. Years of abuse turn her into a bitter, vengeful woman with a holier-than-thou attitude. As soon as she marries Charming, she poisons the King, crowning her husband king. She takes advantage of his weak personality and treats him like a doormat, and spends all of the royal treasure in her own interests.