Well, it could happen if Naruto has some sort of special weapon. For example, he might get a sword that can disrupt Hidan's connection with his voodoo - like powers. Once that connection is broken, Naruto could easily defeat and kill Hidan. Also, Naruto's growth in chakra control and strength could play a role. With his enhanced abilities, he could overpower Hidan and deliver a fatal blow.
One possibility is that Naruto has studied Hidan's abilities thoroughly. He discovers the weakness in Hidan's immortality, which is related to the ritual he performs. Naruto then creates a situation where Hidan is unable to complete the ritual. Without the ritual, Hidan is vulnerable. Naruto takes advantage of this moment and uses his Rasengan or some other powerful attack to kill Hidan. In fanfiction, the author has the freedom to explore different scenarios and character developments that can lead to such an outcome.
Another possibility is a complete character distortion in the fanfiction. The writer might decide to create a 'bad Naruto' version where he has lost all his positive traits. However, this is quite far - fetched considering the strong foundation of Naruto's character in the original series. It would be a very different and probably controversial take on the Naruto - Hinata relationship.
In a Naruto kills Thanos fanfiction, Naruto's sealing techniques could play a major role. He could use sealing jutsu to seal away Thanos' powers or even Thanos himself. Naruto has experience in dealing with powerful foes and sealing them away. Also, Naruto has the ability to sense danger and react quickly. Thanos, on the other hand, may underestimate Naruto at first. By the time he realizes Naruto's true strength, it could be too late for him.
In fanfiction, Naruto might kill a god through the power of his strong will and the unique abilities he has. For example, his Sage Mode could be enhanced to a level that allows him to overpower a god - like entity. Also, his Nine - Tails chakra could be combined with some new, made - up powers in the fanfiction world to give him the strength needed.
Well, in a fanfiction scenario, Naruto might kill Aizen if he manages to use his shadow clones to outwit Aizen's illusions. Naruto could also use his Sage Mode to sense Aizen's reiatsu and counter his attacks. Maybe he combines his Rasengan with some new jutsu he's developed specifically to deal with Aizen's powers.
Another possibility is that there is some sort of magical or otherworldly influence. Perhaps an evil force manipulates Naruto into believing that Drogo is a danger to his world or loved ones. So, Naruto, being the hero he is, takes it upon himself to eliminate the perceived threat. However, this would go against Naruto's usual nature of trying to understand his opponents first, so it would be an interesting twist in the fanfiction.
Another possibility is that the fanfiction is set in an alternate universe where Naruto has a completely different backstory. Perhaps in this AU, he was never loved or accepted, and Christmas, which is often a time of joy and togetherness, became the final straw for him as it emphasized his loneliness and despair.
In the Naruto universe, Naruto killing Neji would be a very dark and out - of - character turn. However, in fanfiction, it could be due to some dark influence like a powerful and evil genjutsu that overpowers Naruto's mind and makes him see Neji as an enemy. Maybe a villain has managed to manipulate Naruto's perception of reality to make him commit such an act.
In a fanfiction scenario where Naruto kills Hanzo the Salamander, it could be due to Naruto's newfound power. Maybe he has mastered a powerful jutsu that gives him the upper hand. For example, if he has a super - charged Rasengan that can break through Hanzo's defenses. Another possibility is that Naruto has allies who help him set up a perfect trap for Hanzo. He could be strategizing with Shikamaru, who comes up with an elaborate plan to outwit Hanzo.
In some fanfictions, Naruto might kill Orochimaru by overpowering him with newfound powers. For example, if Naruto has a sudden boost in his Sage Mode or a new jutsu he developed. Orochimaru is a powerful foe, but Naruto's growth could potentially lead to his defeat. Maybe Naruto has also mastered some sealing techniques that can counter Orochimaru's ability to regenerate and transfer his soul.
If Naruto becomes a Hidan fan in fanfiction, it would be a total shift in his character perception. Naruto has always been about friendship and protecting others, while Hidan is a bloodthirsty and somewhat insane character. Maybe Naruto would see Hidan's fearlessness in battle as something to look up to. But he would also have to grapple with Hidan's immoral actions, like sacrificing people. This could lead to an internal conflict within Naruto as he tries to reconcile his new - found 'admiration' with his own moral code.