In "How To Rock Braces and Glasses", Stevie walks into the bathroom just in time to find that Kacey has rejoined the Perfs and is deeply wounded by the betrayal:
Stevie:(visibly hurt) You people sicken me.
In "How To Rock A Secret Agent", Kacey gets Stevie to infiltrate The Perfs in order to learn what dress Molly will be wearing at an upcoming school dance. Stevie starts Becoming the Mask when Molly is nice to her and remembers essential details about Stevie that Kacey never knew. (Specifically, her middle name, her favorite color, and her birthday.) When Kacey tries to snap Stevie out of it, we get this exchange:
The devastated look on Kacey's face after that remark really seals it.
In "How To Rock Christmas" Kacey and Molly's backstory is revealed. Molly may have done a lot of bad things to Kacey after she became leader of the Perfs, but when you discover what Molly had to go through while Kacey was leader of the Perfs, its actually kinda sad.