Another aspect could be the general discrimination in the BNHA society. The fact that society is so focused on quirks and those without are often looked down upon might have made Izuku bitter. He could be hating everyone because he sees the society as a whole being corrupt and unkind, not just the individuals who directly wronged him.
One possible reason could be that he has had bad experiences with teachers who underestimated him. For example, they might have not recognized his potential and discouraged him from pursuing his dreams, which could lead to his hatred.
The relationships would be completely different. Instead of the usual friendships and rivalries, there would be a lot of hostility. For example, his relationship with Bakugo would be even more strained. Normally, there's a complex dynamic between them, but with Izuku hating everyone, it would be all - out enmity.
One possible reason could be that he feels neglected. Maybe his family was always too busy with other things and didn't give him enough attention. This lack of attention during his formative years could have led to feelings of resentment.
He could hate them because he felt they were absent during his difficult childhood. Naruto faced a lot of loneliness and bullying, and if he thought his parents could have prevented it but didn't, that would breed hatred.
Perhaps in this fanfic, the Weasleys have some actions that conflict with Harry and Hermione's values. For example, if the Weasleys were to make a deal with a dark wizard for the sake of their family's safety and Harry and Hermione found out. This could lead to a rift and hatred. Also, it could be related to a love triangle situation in a very convoluted way. Maybe one of the Weasleys unknowingly gets in the way of Harry and Hermione's relationship, or vice versa, which causes animosity.
One possible reason could be feeling neglected. Maybe his family was always preoccupied with other matters, like protecting the village, and didn't give him enough attention. So he grew to resent them.
One possible reason could be the Ministry's corruption. In the original series, we saw some Ministry officials being more concerned with power and image than doing what was right. They often turned a blind eye to real threats like Voldemort's return in the beginning. So in the fanfiction, this corruption might be exacerbated, leading Harry to hate the Ministry.
One possible reason could be the way he was treated as a child. He was ostracized and shunned by many villagers because of the Nine - Tails inside him. This constant rejection and coldness from the people around him might have led to his hatred in the fanfiction.
One possible reason could be the overwhelming pressure. In the BNHA world, Izuku faces a lot of high - expectations. If he constantly fails to meet them, he might give up. For example, if he fails to control his newfound power or if he keeps getting injured during battles, it could lead to a sense of hopelessness. Another reason could be the loss of important people around him. If he loses someone like All Might, who is a huge inspiration for him, he might lose the motivation to keep going.
It could be due to a misunderstanding. Perhaps there was an incident where his sister was involved, but Naruto misinterpreted her actions as something malicious against him. Like she might have accidentally broken something precious of his and he thought it was on purpose.