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Because Alison has a mentally unstable twin identical sister named Courtney who was pretending to be Ali. Courtney-as-Ali ruined Ali's life, she ditched Naomi and Riley (Ali's bff) and befriend with Aria, Hanna, Spencer and Emily. Meanwhile, the real Alison (who was pretending to be Courtney) had to stay in mental institution because Courtney has mental issues. Alison got tired of it and killed Courtney. She tried to kill the Liars because they had also ruined her life. She didn't want them to be her friends she wanted her old ones.

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13y ago

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