It could also be about narrative tension. Fanfic authors want to keep the story interesting. By having the Avengers be 'dicks' to Tony, they create a source of conflict. This conflict can drive the story forward, whether it's leading to Tony leaving the team for a while, or him trying to fix the relationships. It gives the author more room to explore different emotions and plot developments, like Tony's journey to regain the trust and respect of his fellow Avengers.
In some fanfics, the Avengers might be 'dicks' to Tony due to jealousy. Maybe Tony's intelligence and wealth make them feel inferior in some ways. His high - tech gadgets and his popularity could stir up envy among the other Avengers, leading to unkind treatment in the fictional stories.
In fanfic, sometimes the writers want to create drama. By making the Avengers assholes to Tony, they can build up conflict. It could also be related to past events that the fanfic author has created. Maybe Tony made a mistake in the past that the other Avengers can't forgive, even though it was an accident or done with good intentions, and they keep treating him badly.
He might dislike it because it can create false expectations. Fans read fanfiction and might start expecting the real Avengers to act in a certain way that is not in line with their true nature.
Differences in ideology can also play a part. Tony might have some views on how to handle threats or use technology that the other Avengers don't agree with. Maybe he's more willing to take risks with untested tech that could endanger the team or civilians, which would lead to resentment.
One reason could be to create a unique love story. Writers like to take unexpected pairings to make their fanfics stand out.
One reason could be to explore the extreme consequences of their ideological differences. In the official story, they had disputes over things like the Sokovia Accords. In fanfiction, this could be pushed to the limit, leading to Tony taking Steve's life.
One reason could be to explore the darker side of the characters. Some fans like to see how the Avengers could be pushed to their limits and do something out of character, like hurting Tony as a teenager. Another reason might be to create a more angsty and emotional story, with Tony as a victim and the Avengers as the antagonists, which can add a lot of drama.
One reason could be that he's frustrated with the leadership or the direction the Avengers are taking in the fanfiction. He might feel his ideas are not being heard.
One possible reason could be his excessive exposure to the arc reactor technology. The energy from the arc reactor might have gradually altered his DNA over time, causing him to become something other than human.
Maybe in the fanfiction, the other Avengers are seen as equally responsible. So, Tony doesn't bear all the blame for Ultron.