Warning: SPOILERS ahead for 13 Reasons Why season 3.
13 Reasons Why season 3's biggest twist wasn't Bryce Walker's murder but rather who was ultimately blamed for his death and who died himself: Monty. The first two seasons of 13 Reasons Why focused on Hannah Baker's suicide and the tapes she made right before her death, revealing her exact reasons for wanting to die. After that, it became about getting justice for Hannah's death, particularly going after Bryce, who raped her and several other girls at Liberty High School.
Since Bryce effectively got away scot-free at the end of 13 Reasons Why season 2, the major focus of 13 Reasons Why season 3 was finding out who murdered him. It was shocking to find out that Bryce was murdered before season 3 even released, but overall not too surprising given everything he had done in the first two seasons. And so, taking everything into account, the police accused Clay of murdering Bryce, but in the end, it was Monty who went down for the crime (after already being in jail for sexually assaulting Tyler in season 2).
In the 13 Reasons Why season 3 finale, Zach reveals that Tim Pozzi was in county lockup when Monty was killed in his cell (it's implied that he was beaten to death, possibly for being gay, as his father said could happen). Tim was released and told Charlie, who then told Luke. It spiraled from there, with Luke telling Justin and Justin telling Ani, who devised a plan to use Monty as a scapegoat for Bryce's murder since he was already dead.
Monty, like everyone else, had a clear motive for wanting Bryce dead, and that's what was eventually used against him, even after his death. Bryce threatened to turn in Monty to the police, making everything bad that he jock once did public. Monty thought he and Bryce were brothers, so Bryce laying out all his cards felt like a betrayal to Monty. And given that Monty couldn't account for his whereabouts since he wouldn't come to terms with the fact that he was gay and say that he was with Hillcrest's Winston Williams, his lack of an alibi was suspicious.
But the thing is, this lie may not hold together in the long run, as 13 Reasons Why's season 3 ending indicates that the truth may come out about who Monty was with when Bryce died, thereby proving that it was all a lie. There's also the fact that Deputy Standall suspects (and may now know) that his son, Alex, was, in fact, the one who killed Bryce. But like many parents, he would go to great lengths to protect his family. In the end, 13 Reasons Why season 3's biggest twists weren't about Bryce but rather about Monty - that he was gay and his shocking death.