Perhaps in the story, Izuku has had experiences where adults didn't keep their promises to him. This repeated pattern of broken promises led to his distrust. He may have expected certain things from adults, like protection or guidance, but they didn't deliver, so he became distrustful.
It might make him more hesitant to form close relationships with adult characters. He may be guarded and not open up easily to them.
One possible reason could be that in the original story, many adults failed him. For example, Dumbledore, while having good intentions, often kept Harry in the dark about crucial matters. In fanfiction, this could be amplified. Harry might have seen adults make decisions that put him and his friends in danger without consulting them. Another reason could be that some adults in the wizarding world have their own political or personal agendas that conflict with what Harry believes is right, so he doesn't trust them.
Maybe Harry discovers some things from Dumbledore's past that seem unethical to him. For example, Dumbledore's association with Grindelwald in his youth. Harry, who has a strong sense of right and wrong, might find it hard to trust someone with such a controversial past. And in the fanfiction, the author might explore how these revelations shake Harry's faith in Dumbledore.
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.
Sometimes, the source material itself can be a factor. If there are major changes in the official My Hero Academia storyline related to Izuku, it might make the author's original concept for the fanfiction no longer viable. Or perhaps they had planned the fanfiction to be part of a certain arc in the series, and when that arc didn't play out as they expected in the official story, they abandoned their work.
It could be due to Bakugou's public shaming of Izuku. If he embarrasses Izuku in front of others too often, Izuku will reach his limit and snap.
One reason could be stress. Izuku might be under a lot of pressure to keep up with his training and the expectations of being a hero. Class 1A's constant high - energy and sometimes chaotic behavior could push him over the edge. Another reason could be misunderstandings. If they misinterpret his actions or motives, he might snap in frustration. For example, if they think he's showing off when he's actually just trying to help.
He would likely be more confident. Having the powers of the Human Torch gives him a new level of strength. So he might stand up straighter and be more assertive in his actions.
One possible reason could be that Izuku realizes Bakugou's unchanging and hostile attitude. Despite his efforts to be friends or get along, if Bakugou continuously rejects him or is overly aggressive, Izuku might feel it's a lost cause.
One reason could be that Izuku has been pushed to his limit. Maybe Aizawa has been overly critical or strict in a way that Izuku can't take anymore. For example, if Aizawa constantly doubts Izuku's abilities despite his efforts, it could lead to Izuku snapping.