Warning: Spoilers from 13 Reasons Why season 3's finale are ahead.
The conclusion of the murder mystery at the center of season 3 of 13 Reasons Why can be summed up with one phrase: "Let the dead bury the dead." What the sentence that Ani (Grace Saif) tells Deputy Standall (Mark Pellegrino) means sums up the way these characters finally solve the problem they've been saddled with ever since the body of Bryce (Justin Prentice) was discovered.
Bryce's murderer is finally revealed in the season 3 finale of 13 Reasons Why. Though Ani tells Deputy Standall it was Monty (Timothy Granaderos) who was Bryce's killer, the person really responsible for Bryce's death was Alex (Miles Heizer). He killed Bryce by pushing him off a dock and into the river to drown, knowing full well that Bryce would be unable to swim after Zach (Ross Butler) broke his leg. The reason? Bryce threatened Jessica (Alisha Boe), who was at the dock with Alex and witnessed the whole thing.
Deputy Standall suspects it was his son Alex who killed Bryce, after learning about Alex's steroid use and realizing that tire tracks consistent with Alex's car were found at the crime scene. Deputy Standall would do anything to save his son, and Ani anything to make sure her friends stayed out of prison. So, Ani weaves a story: It was Monty who followed Bryce to the docks that day, not Alex. She has Charlie (Tyler Barnhardt) plant the tape Bryce made for Jessica, which had Bryce's blood on it, in Monty's locker, and has Tyler (Devin Druid) supply Alex with an alibi. Knowing that Monty's whereabouts the night of Bryce's death are unknown, and that he's already in prison for sexual assaulting Tyler, Ani's story is totally plausible.
What Ani didn't know at the time of telling Deputy Standall this story was that Monty was just killed in prison.
"I'm sorry that Monty's dead. I am. Justin tells me he could be loyal, and fearless, and he stood up for his friends. I'm sorry that Bryce is dead. He was my friend and I believe he would have showed the world that redemption is possible. But they both caused pain when they were alive. They caused great pain to a number of people. It is time for that suffering to end. So let the dead bury the dead, Deputy Standall. Let the dead bury the dead."
The phrase "let the dead bury the dead" comes from a story in the Bible. In the story, Jesus meets a man who claims he wishes to go with Jesus, but says he must bury his father (who is still alive) before he does. Jesus says "Follow me, and leave the dead to bury their own dead," meaning he wants the man not to fulfill his spiritual duties instead of staying with his father, whom Jesus believes is "dead" in a spiritual sense. Today, it is often used by people a way to say "forget the past, what's done is done."
Here, however, it has a more practical meaning for 13 Reasons Why: Let Monty take responsibility for Bryce, as both Monty and Bryce are dead. The pain they caused is enough reason to make this the moral decision.
Whether that's true is up for debate. It also may not work. Winston (Deaken Bluman), the student from Hillcrest who Monty hooked up with the night of the homecoming game, finds Ani after Bryce's case is closed to tell her that he was with Monty when he was allegedly murdering Bryce.