His turn to evil could be due to a corruption of his sense of justice. If he saw too much injustice in the world that couldn't be fixed by the normal means of being a friendly neighborhood Spider - Man, he might decide to take matters into his own hands in a more extreme and unethical way. He could start to believe that only through fear and force can he really make a difference, which would be a significant departure from his original heroic values.
One possible reason could be extreme grief. If he lost someone very close to him, like Aunt May in a particularly cruel way, it might shatter his moral compass. Another reason could be mind control. Maybe a powerful villain manages to get into his head and twist his thoughts and values. Also, if he was constantly betrayed by the people he trusted, such as the Avengers or his friends at school, he could gradually turn bitter and evil.
His relationship with MJ would be completely shattered. Instead of being the loving boyfriend, he could become possessive or even abusive. His friends at school would be terrified of him. Ned, who was once his closest friend, would probably try to avoid him or even try to find a way to stop him. And his relationship with the Avengers would turn into enmity. They would see him as a threat and would try to contain or stop him.
One possible reason could be a misunderstanding. Maybe Peter has been acting secretively about his Spider - Man activities, and his team misinterprets it as him being up to something bad. For example, if he has to disappear suddenly to fight crime, his team might think he's ditching them for other, less noble reasons.
One possible reason could be extreme exhaustion. Peter Parker has a double life as a student and Spider - Man. He might be overworked, not getting enough sleep, and his body just gives out. For example, he could have been fighting multiple villains in a row without a break.
One possible plot could be that Peter Parker's transformation into a spider is a permanent one. He has to deal with not just his regular superhero duties but also the new challenges of being part - spider all the time. Maybe he has to learn new ways to communicate with other spiders to gain some special knowledge or power.
One possible reason could be a difference in ideology. Maybe Peter Parker feels that Stark's methods are too extreme or not in line with his own moral compass. For example, Stark might be involved in some projects that Peter doesn't agree with, such as using certain unethical technologies in the pursuit of justice.
In some fanfiction scenarios, Peter might be dealing with some sort of internal power struggle. If his spider - powers are evolving or being affected in some way, it could lead to his collapse at school. For instance, if there's a glitch in his powers that causes his body to have sudden energy drains or physical reactions, like collapsing, while he's at school trying to act normal.
One possible reason could be extreme stress. Peter Parker has a lot on his plate, being Spider - Man and having a normal life. If in the fanfiction he faces a series of really tough situations like failing to save someone important or being hunted by a very powerful enemy, the stress might get to him and lead to self - harm as a way of coping. Another reason could be feelings of isolation. Even though he has friends, he might feel like no one truly understands the dual life he leads, so in the fanfiction this could drive him to such a desperate act.
One reason could be a fight with a powerful enemy. Maybe a new super - villain has emerged in the city and Peter underestimated their strength during the battle in the alley.
In some fanfictions, it could be that he has made a big mistake. For example, if his actions as Spider - Man led to the harm of an innocent person. He would be so guilt - ridden that he decides to run away from everything, including his friends and family in Queens.
Well, one reason could be that he's on a secret mission for S.H.I.E.L.D. that requires him to go undercover and cut off all contact. Maybe he's investigating a new super - villain threat that's too dangerous to let anyone know about.