Another possible motive could be revenge. Maybe something the Uchiha clan did in the past, that Naruto discovers later, makes him angry enough to take such drastic action. However, this would be a huge deviation from his normal character, which is what makes this fanfiction so interesting in exploring the 'what - ifs' of his personality.
His motive could be a false sense of loyalty to the village. If he believes that the Uchiha clan's actions, real or perceived, threaten the safety and stability of the village, he might think that massacring them is the only solution. This would show a more extreme and dark side of Naruto's character, where his loyalty to the village overrides his sense of right and wrong.
One possible motive could be mind control. Maybe an enemy used a powerful genjutsu on Naruto, forcing him to commit this heinous act against the Uchiha clan.
One possible motive could be that Naruto is being controlled by some dark force or genjutsu. Maybe an enemy has manipulated his mind to target Akamaru in order to get to Kiba. Another reason could be a misunderstanding. Naruto might think Akamaru has been infected with something dangerous and he's trying to keep it away from others by holding it hostage, but in a very wrong way.
One possible motive could be a major misunderstanding. Maybe Naruto was under some kind of mind - control or false information that made him believe the Uchiha clan was a threat to the village on a much greater scale than they actually were. So, in an attempt to protect the village, he took extreme action.
One possible motive could be to gain control over Naruto's power. Since Naruto has the Nine - Tails within him, Danzo might want to use that power for his own political or military gains. He's always been power - hungry and sees Naruto as a tool to achieve his goals of making Konoha stronger in his own way, which might involve more underhanded and controlling methods.
It could be a situation where Naruto discovers some dark secret about the Uchiha clan that he believes makes them a danger to everyone. For instance, if he found out they were planning a coup that would involve mass destruction, he might think that killing them was the only way to save the village and the rest of the ninja world.
If Naruto saved the Uchiha clan, the story of the Hidden Leaf Village would be rewritten. The history of the Uchiha's massacre would no longer be a dark stain on the village's past. The Uchiha would contribute their unique skills and abilities to the village's development. For example, their Sharingan abilities could be used for better security or in battles against common enemies. There would also be a change in the relationships between characters. People who were once enemies of the Uchiha might have to learn to work with them.
One way Naruto could save the Uchiha clan in fanfiction is by using his influence. Naruto is highly respected in the village. He could convince the village elders to re - evaluate their treatment of the Uchiha. He might show them that the Uchiha were being wrongly persecuted and that there was a way to co - exist peacefully. He could also use his power as a Jinchuuriki to protect the Uchiha from any external threats that might be a factor in their downfall.
One possible motive could be revenge. Maybe Naruto was led to believe that the Namikaze clan was responsible for something terrible that happened to him or someone he cared about. Another motive could be power. If there was some sort of power within the clan that Naruto thought he could gain by eliminating them.
Well, one possibility is that Naruto would be trained in the unique Uchiha techniques. He might learn how to use the Sharingan in ways that no one expected. Also, he could gain a new family within the Uchiha clan, which would change his whole perspective on life and belonging.
In the fanfiction, it could be that Naruto has had some personal negative experiences with Uchiha clan members. He might have witnessed some Uchiha using their power to bully or oppress others. Naruto, being the kind - hearted person he is, would not tolerate such behavior and thus develop a hatred for the clan as a whole. This could also be related to the Uchiha's reputation for being a bit of an outcast clan, and Naruto might see them as not fitting into the harmonious village life he envisions.