Detailed Explanation
Phoebe's death is the key to understanding everything. Cora's sister was chronically ill but desperate to experience life. Cora took her to a party run by drug dealers, where things went horribly wrong. Phoebe overdosed and died. Cora was drugged and assaulted. The trauma was so severe that Cora's mind created a false narrative—she believed Phoebe died of her illness at home. The truth only emerges through Ambrose's investigation, revealing that Cora's seemingly random violence had deep roots in buried trauma.