A Harry Potter fan theory suggests that Nagini may have chosen to join Voldemort’s cause and become a Horcrux for a reason.
The Fantastic Beast and Where to Find Them films were created with the goal of illuminating parts of the Wizarding World’s history that were left out of the Harry Potter series. That included, at least in one of the films, delving deeper into the origins of a pivotal villain. In the Fantastic Beasts movie, Nagini’s sources hint at her gloomy future as Voldemort’s animal sidekick. However, her plot made no progress in the most recent installment of the series, The Secrets of Dumbledore. Instead, fans have taken it upon themselves to explain how the once-heroic Nagini had turned criminal by the time of the Harry Potter books.
As revealed in the Fantastic Beasts and Where to Find Them: The Crimes of Grindelwald, Nagini was born in a humanoid form. Nagini was a Maledictus with a blood curse who could turn into a snake and was doomed to be locked in that reptilian body unless she found a cure. Nagini was a member of a magical circus during the interwar years and had a quasi-romantic relationship with another performer, Credence. Nagini grew to care for him and attempted to assist him in escaping and discovering his true identity.
By the end of the film, Nagini had attempted to assist Dumbledore and his allies in saving the Wizarding World from Grindelwald — and, in the process, to save Credence from the dark wizard’s grasp. Although Nagini does not appear in the following film, her fate was always doomed to be disastrous. Nagini, who’d become trapped in her snake form, had been discovered and recruited by a new Dark Lord, Voldemort, by the time of the main Harry Potter series later in the twentieth century. Voldemort most certainly found her and persuaded her to join him, eventually changing her into one of his Horcruxes, thanks to his ability to speak Parseltongue.
It’s not unexpected that Nagini was left out of The Secrets of Dumbledore, given the difficulties of turning one of the franchise’s few Asian characters into an evil beast. However, it raises various additional issues about how and why Nagini would ever become a servant of evil. According to a Reddit user named herald zemo22, Nagini turns against the more heroic wizards for a specific reason: she wants retribution. Credence was a member of the Dumbledore family, according to Secrets of Dumbledore, and was the son of Aberforth, making him Albus’ nephew. Credence is finally welcomed by his family and returned home at the end of the film, but he is already on the point of death, and the Dumbledores seem unable to help him in his dying days.
According to this theory, Nagini’s absence from Secrets of Dumbledore could be due to a mission to find Credence. On the other hand, Nagini would be enraged by the Dumbledores’ seeming indifference to Credence’s plight when she learned of his death. Years later, Nagini might be more inclined to listen if she knew that a new Dark Lord was plotting against the Wizarding World, especially if it meant targeting Dumbledore. It’s plausible that Nagini sought Voldemort herself, considering the Dark Lord as a potential ally in her quest for vengeance against Dumbledore for failing to preserve Credence.
This concept puts someone who has lost their humanity into a literal story of a hero who has gone wrong, implying that Nagini accepted her fate as part of a plan to avenge Dumbledore’s previous failings. In a universe where atonement is the norm, it gives her a legitimately compelling history. It’s uncertain if Nagini’s story will ever be fully resolved, but if the powers that be addressed her journey into evil in this fashion, they may at least give her a uniquely sad edge.