Maybe Riser made a deal with some of Naruto's enemies. Let's say he allied with Orochimaru to get some powerful jutsu. Naruto discovers this and sees Riser as a traitor to the world of ninjas. In Naruto's view, those who side with the likes of Orochimaru are a great danger, so he decides to take Riser out to prevent any further harm.
One possible reason could be a power struggle. If Riser somehow poses a threat to Naruto's world or his friends. Naruto, being the protector, might take extreme measures. For example, if Riser was trying to take over Konoha or steal some important ninja art. Naruto would fight to defend his village and in the heat of battle, end up killing Riser.
It could also be a misunderstanding. Riser might be doing something that Naruto misinterprets as a harmful act. For instance, Riser could be searching for a powerful artifact in Naruto's world for a good reason, but Naruto thinks he's going to use it for evil. So, Naruto attacks and kills him without fully understanding Riser's intentions.
One possible reason could be that Root has some evil plans that directly threaten Naruto's friends or the village. Naruto, being the protector, has no choice but to take them down. For example, if Root was planning to assassinate the Hokage and Naruto found out, he would fight back and might end up killing some of their members.
One possible reason could be a misunderstanding. Maybe Sasuke is framed for something really bad, and Naruto, in his anger and sense of justice, lashes out without knowing the truth. For example, if there's a false rumor that Sasuke has attacked the village again, Naruto might be quick to act.
Well, one possible plot could be that Naruto is somehow transported to the world where Riser exists. Naruto, with his strong will and powerful jutsu, might see Riser's actions as unjust and decide to oppose him. Maybe Riser is causing trouble in a village or harming innocent people, and Naruto steps in to protect them.
Maybe in the fanfiction, Naruto is reincarnated as Riser Phenex in the Naruto world. He has to deal with the memories and instincts of Riser while also trying to maintain his identity as Naruto. He could use Riser's abilities, like his fire - based powers and his unique magic, to fight against new threats in the Naruto universe and also to protect his friends and village.
In fanfiction, anything is possible. Naruto has his powerful jutsu like Rasengan and Shadow Clone Jutsu. If he were in the Highschool DxD universe, he might use his speed and surprise factor. Maybe he could catch Riser off - guard and land a fatal blow with a Rasengan. However, Riser also has his own set of powers in the Highschool DxD world. So it would be a very interesting and intense battle if written well in a fanfiction.
Perhaps in this fanfic, Tsunade kills Naruto out of desperation to save the village from a perceived greater threat. If there was a prophecy that Naruto's power would lead to the destruction of Konoha in a certain way, Tsunade, in her role as Hokage, might feel it her duty to stop him at all costs. This would be a very tragic turn as Naruto is seen as a hero in the original story.
Well, if Naruto becomes a Riser in a fanfiction, the plot could change drastically. Maybe Naruto would inherit Riser's powers which are related to fire and some sort of devilish abilities. He might start to have a different view of the ninja world. Instead of the traditional ninja values he grew up with, he could be more influenced by the power - hungry nature that might come with Riser's powers.
Well, it could be about Naruto being in a strange new world where he is somehow related to Riser. Maybe they team up to fight common enemies, with Naruto bringing his ninja skills and Riser his own unique abilities.
One possible reason could be a misunderstanding during a battle. Maybe there was some sort of jutsu misfire or misinterpretation of an attack. For example, if Kiba was using a new and powerful technique that Naruto didn't recognize, Naruto might instinctively go all - out in self - defense, nearly killing Kiba.
Naruto could also become more determined to avoid such situations in the future. He may work harder on his diplomatic skills or try to understand his enemies better before jumping into a fight. This experience could be a turning point for him, making him grow as a ninja not just in terms of power but also in wisdom and self - understanding.