The penultimate episode of The Sinner gave fans a lot of the answers they craved about July 3, 2012 — including providing a motive for Cora killing Frankie on the beach five years later. But it isn't until the end of the miniseries where viewers discover who had kidnapped Cora on The Sinner, and why she couldn't remember that night. Spoilers for The Sinner finale follow. Turns out, Frankie's father is the masked man who abused Cora during those months when she was missing. And with Frankie's own parents responsible for Cora's repressed memories, The Sinner's conclusion shows the power of abuse while nearly absolving Cora of all guilt.
Even seeing how the night of July 3 went down during "Part VII" didn't fully explain why Cora would kill Frankie to avenge her sister's death. But the twist reveals that it was Frankie's father who kept her drugged for two months and then dumped her body in order to ensure his son's future career as a doctor, and everything adds up. Although Cora is guilty of murdering Frankie, she's also the victim. Frankie's father forced Cora to use heroin so that she wouldn't be able to form memories of where she was being held, which almost negates Cora's responsibility in his son's eventual death. The abuse perpetrated on her took away her agency and deeply suppressed the horrific memory of her sister dying in Cora's mind. That suppressed memory was triggered when she heard the song that was playing during Phoebe's death again on the beach. And the abuse inflicted on her was so powerful that it led Cora to resort to her instincts, which were to recreate how she had hit Frankie when her sister died — but this time, with a knife.
If she had been able to wake up in the hospital after being hit in the head by J.D. and be told that Phoebe had died, Cora could have processed her complicated emotions of grief, anger, and guilt then. Instead, the Belmonts took away part of her life by making her a drug addict. To further cover up the crime, Frankie's father and J.D. buried Phoebe in an unmarked grave in the woods and Frankie's father allowed J.D. to blackmail him into providing the prescriptions for his drug dealing business. Then, to top it all off, the Belmonts not only allowed Cora to be tried for the murder of their son, but they called her a liar and blamed her without ever once revealing the truth.
Frankie's parents were underdeveloped throughout the miniseries, so having them turn out to be the monsters behind her memory lapse almost feels like a cop-out. But their actions also neatly tie up the story of The Sinner by confirming that Cora is a fully sympathetic character. Frankie himself seemed like a decent person — you know, except for the fact that he was willing to listen to his dad and take no responsibility for his actions — and didn't deserve to die. But Frankie's father and mother (let's not ignore her culpability in the situation) only cared about their son's future, not Cora's life. And their selfish actions unexpectedly but directly made them responsible for essentially creating their son's murderer. Because while Cora isn't a killer, as she says in her statement to the judge during her original hearing, five years of never knowing the truth momentarily turned her into one.
The coverup of Phoebe's death caused so much more destruction than if the Belmonts had just confessed. While The Sinner finale doesn't say what will become of Frankie's parents, the judge thankfully reconsiders Cora's sentence because of "extreme emotional disturbance" and gives her time in a psychiatric facility instead of prison to be evaluated every two years. That means the same abuse that caused Cora to kill Frankly is what eventually gives her her life back. As her mother's Bible says, "The truth will set you free."