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.
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 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.
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.
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.
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.
One possible reason could be a misunderstanding. Maybe Percy did something that Annabeth misinterpreted, like making a decision during a quest without consulting her first, which made her feel disrespected.
One great 'bnha cool izuku fanfic' is 'Deku: The Prodigy'. It shows Izuku in a really cool light, with unique character development and exciting plot twists. Another one could be 'Izuku's Ascent' which focuses on his journey to becoming an even greater hero than in the original story, with some really cool new powers and abilities.
It could be that the fanfic portrays Hinata as being arrogant towards Naruto's past. Naruto has had a difficult childhood and if Hinata in the fanfic mocks or belittles that, it would make Naruto hate her. For instance, if she scoffs at his time as an outcast in the village and uses it as a means to elevate herself, Naruto would surely despise her for it.
One possible reason could be a misunderstanding. Maybe Lupin did something that Harry misinterpreted as betrayal or negligence. For example, if Lupin was forced to keep a secret from Harry for his own safety, but Harry found out in a way that made him feel deceived.
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.