Motivation can be shown through a character's actions. For instance, if a character is motivated by revenge, they might constantly train to get stronger and seek out their enemy. Their dialogue can also reveal motivation. If they talk about their goals often, like protecting the village, it shows what drives them. Another way is through their reactions to events. A character motivated by friendship will go to great lengths to help their friends when they're in trouble.
One possible motivation could be power. Orochimaru is always seeking more power, and Naruto has the power of the Nine - Tails. He might think that by killing Naruto and somehow harnessing the Nine - Tails' power, he could become even stronger.
He might be desperate. Maybe there is a great threat to the ninja world that can only be defeated by the power of the Shinju. Naruto, being the hero he is, decides to take the risk of eating it in order to gain the power needed to save everyone.
One possible motivation could be mind control. Maybe an enemy uses some powerful jutsu to take over Naruto's mind and make him do it against his will. However, this would be a very dark and extreme take on the Naruto universe.
He could have been exposed to some dark secrets about the ninja world that made him lose faith in the traditional ways. Madara had his own reasons for his actions, and if Naruto discovers similar things, he might start acting like him. For example, if he finds out that the villages have been using ninjas in unfair or unethical ways for a long time, he could turn to more extreme measures like Madara did.
One possible motivation could be a misdirected sense of revenge. Maybe Naruto is tricked into believing that Iruka has done something unforgivable against Konoha or someone he loves. But this would require a huge distortion of the characters' personalities.
The bullies could also be influenced by their parents or the village's general perception of Naruto. If the adults in the village see Naruto as a troublemaker or a potential danger (maybe because of the Nine - Tails within him), the kids might pick up on that and bully him as a way to fit in with the adults' views.
It could be a symbolic way of showing the destruction of hope. In the Naruto series, Naruto represents hope. By having Hinata kill him, the writer might be trying to convey a sense of hopelessness or the end of an era. This could be part of a larger, more complex story about the fall of the ninja world as we know it in the Naruto franchise.
There could be several reasons. One might be that they simply have a dislike for the Naruto franchise and are using this extreme concept as a form of protest in a very unorthodox way. Another possibility is that they are just exploring some really dark and unethical ideas for the sake of creativity, without considering the negative impact it could have on the fans of the series. It's also possible that they don't fully understand the significance and positive influence of the Naruto character and are just making a random and inappropriate statement through this fanfiction.