Warning: the following contains SPOILER for Trollhunters: Rise of the Titans.
Trollhunters: Rise of the Titans ends with a shocking twist, but ironically enough, it's replaying the plot from a season 2 episode of Trollhunters: Tales of Arcadia, only this time the lesson is reversed. The new Trollhunters movie is the culmination of the three stories in Guillermo del Toro's animated Netflix shared universe, and it's full of call-backs and references, including the ending, which, at first glance, seems to subvert a lesson Jim learned previously.
After everything that happens over the course of the Trollhunters franchise, Trollhunters: Rise of the Titans ends with Jim Lake and his friends winning the day, but with a few big losses, particularly his best friend Toby. So, when he has the opportunity to start over and utilize his memories to make a better timeline for him, his friends, the decimated Arcadia Oaks, and the rest of the world, he takes it, making some key immediate changes to the timeline when he gets there, most notably letting Toby become the Trollhunter instead.
Ironically, this isn't the first time Jim has had the opportunity to relive this day and turn down the amulet. In Merlyn, the creator of the amulet. But just as Jim charges off to meet Gunmar, and certain doom, he wakes back up, still the Trollhunter, steeling his resolve to take the full burden of being the Trollhunter.
Given the lesson of this ordeal was "I'm the Trollhunter, amulet or not," it's a little odd for him to then decline it when he goes back to fix the timeline, but in this new context, with Jim having developed even further as a character, the decision is no longer one of defeat or surrender, but evolution, while also providing a chance for Toby to become the kind of hero he always wanted to be. Jim may have surrendered the actual Trollhunter power, but the knowledge he's gained makes him an invaluable mentor for Toby and enables him to be a leader, organizing the team of Trollhunters and preparing them to face the threats of this new timeline in a way they couldn't have before.
It's not clear if the inversion of "The Unbecoming" was intentional, but it adds an interesting layer of depth to both Jim and Toby's hero's journeys, especially considering the titles of both "The Becoming" and "The Unbecoming" reference the speech Blinky gives Jim, which would become the monologue Jim uses to audition for Romeo and Juliet, later serving as a tribute to Jim's original voice actor, Anton Yelchin, and becoming the mantra for the whole franchise at the end of Trollhunters: Rise of the Titans: "Destiny is a gift some go their entire lives, living existences of quiet desperation, never learning the truth that what feels as though a burden pushing down upon our shoulders is actually a sense of purpose that lifts us to greater heights. Never forget that fear is but the precursor to valor, that to strive and triumph in the face of fear is what it means to be a hero. Don't think, become." Through the course of Jim's journey, he's become a hero, and he no longer needs the amulet to serve that role, enabling Toby to also rise up and "become."
