I think one of the things that really make this story is that Annabelle isn't a stereotypical preppy bitch, she's popular but she seems really nice too. In that same regards Kate isn't antagonistic with her, she considers her a friend, her best friend. The idea that what started as a joke became real is enticing, Kate only seemed to play around with the idea at first but as she went on she became more and more consumed with jealousy, turning her friend into a fat nerd. The internal struggle and transformation of both characters really make this story.
The fact that Kate knows what she's doing to Anna on some level but don't care, in fact loving the new changes, shows that on the inside she's already changed. That secne where Kate is feeding Annabelle the pasta and reinforcing her as "Anna" and the part where she was talking to her brother getting a feel of what she was going to turn Annabelle into were my favorite parts.