It could be because of the way Naruto's power sometimes goes out of control. When the Nine - Tails' chakra leaks out, it can be quite intimidating. Sakura, not knowing how to handle it or being scared of it, might lash out and call Naruto a demon. Also, in the early days, Naruto was often ostracized, and Sakura may have been influenced by the general perception of Naruto as something 'other' or dangerous.
Well, it could be that in this fanfiction, Naruto has some sort of hidden agenda. He might call Sakura ugly as a form of reverse psychology. He could be thinking that by doing so, she'll prove him wrong and become more confident. Or perhaps there's some miscommunication or misunderstanding between them. For example, Naruto misheard someone talking about Sakura in a negative way regarding her looks and he wrongly believes it, so he says she's ugly.
Maybe he's frustrated with her lack of progress in combat compared to him and Sasuke. He could also be in a bad mood and say it without really meaning it.
One possible reason could be Naruto's out - of - control power. If in the fanfiction Naruto has some new and extremely powerful abilities that he can't fully manage, Sakura might see him as a monster out of fear. For example, if he accidentally causes destruction while trying to save the village.
In a half - demon Naruto fanfiction, Sakura might also have to adapt to Naruto's new powers. This could create a new dynamic where Naruto is sometimes the one in need of her help and sometimes the one protecting her with his enhanced abilities. Their relationship would be more complex, with a mix of care, respect, and a new - found understanding of each other's roles in their shared journey.
A common villain could be Orochimaru. Since he was always interested in powerful jutsus and abilities, he might target Sakura to use her as a pawn in his experiments. And Naruto, with his demon powers, would have to stop him. Orochimaru is a well - known and menacing character, so it makes sense for him to be a villain in these fanfictions.
Another plot might involve Naruto's demon side being awakened in a different way than in the original story. He discovers that his demon powers are the key to saving Sakura from a curse that was placed on her. So, he has to learn to control and use these new - found powers to break the curse and free her.
It might also cause Naruto to lose control more easily. If he hears such a hurtful term from someone he cares about like Sakura, it could make him more vulnerable to the Nine - Tails' influence. In the long run, it could be a turning point in his character development, either making him more bitter or more determined to show everyone, including Sakura, that he's not what she says.
A possible plot is that a group of evil ninjas are hunting half - demons, and Naruto is in their crosshairs. Sakura vows to protect Naruto at all costs. They go on the run together, and during this time, Sakura discovers that she has a special connection to Naruto's half - demon side. Maybe she has some long - lost family connection to the same demon race, which gives her unique abilities to help Naruto.