Simone was also the girlfriend of Justin Stiles, a popular member of Shadyside's baseball team who also had a habit of seeing other girls. Enraged at Justin for his betrayals, Simone plotted to get revenge on her unfaithful boyfriend--by killing every girl he had been with. Simone began her plot by faking her own death, ransacking her room and putting her own blood on the floor, later running away from the house wearing a hat and baseball jersey, allowing Lizzy (who discovered the staged scene) to believe Simone's killer was a baseball player.
Now with the police believing an active serial killer was behind her murder, Simone began killing off Justin's other dates, who also happened to be her fellow Prom Queen candidates. Simone's first victim was Rachel, who she stabbed to death in her bedroom. The evil Simone also attacked Dawn while she and her friends were out at the movie theater by punching her in the face before killing Elana, bludgeoning her to death in the school auditorium. Simone continued wearing a baseball jersey during her crimes, allowing Lizzy to first believe Lucas Brown (Simone's ex-boyfriend) and then Justin of being the killer.
In the book's climax, Lizzy and Dawn (who were working on their school's production of The Sound of Music together) were in the auditorium late after a dress rehearsal, where Simone appeared and stabbed Dawn with her knife (with Dawn ultimately surviving due to wearing Lizzy's bomber jacket). Simone then revealed herself to Lizzy, who was shocked to see that Simone was both alive and the true killer. Simone then ranted to Lizzy about how she had been killing all the girls Justin went out with behind her back to get back at him for hurting her, while proclaiming that no one cared about her and refusing to believe Lizzy's claim that she hadn't been out with Justin and had rejected his invitation to go to Prom together. Simone then approached Lizzy with her knife raised, with Lizzy briefly subduing the villainess by dropping a sandbag on her foot, breaking it. But as Lizzy checked on the wounded Dawn, Simone attacked Lizzy again and tried to strangle her, only to be defeated when Dawn stabbed her in the leg with her knife and Lizzy tackled and held Simone down when she tried to attack Dawn again. The police arrived soon afterwards and Simone was presumably arrested.
- "My parents never cared about me. All they cared about were their golf scores and their martinis. Justin didn't care, either. He just used me. Nobody cared. Nobody." (Simone delusionally claiming nobody cared about her)
- (Lizzy McVay: "Okay. He asked me. But I said no.") Is that so? Now, why don't I believe that for a single second? You're next, Lizzy. Sorry. I thought you were my friend, but you were only pretending. You didn't care about me, either. You're just like all the others." (Simone proclaiming that Lizzy was lying about not going out with Justin and that she was just like everyone else in her life, right before attempting to kill her)