I just realized, there has never really been an evil she-cat. NO, Bluestar was not evil, she was just angry and depressed. And Hollyleaf wasn't really evil, just crazy.
Victoria Holmes: Meredith, your wish for a purely evil she-cat will come true in OTS3 NightWhispers. Behold...MAPLESHADE! - Victoria Holmes' Facebook Page
According to Vicky, Mapleshade is a very large white tortoiseshell with amber eyes. She is ambitious, cold, and craves revenge. More will be revealed by Mapleshade in Crookedstar's Promise. She is also said to be a member of The Place of No Stars. This means that she is dead in Night Whispers, but probably alive in Crookedstar's Promise.