Well, it could be that in this fanfiction, Snape is written in a way that endangers Harry's friends. Harry is very protective of his friends, so if Snape is portrayed as harming them in some way, Harry would likely try to destroy whatever it is that's causing the threat. Also, it could be related to some prophecy or hidden knowledge in the fanfiction that makes Harry believe Snape has to be stopped.
One possible reason could be a dark turn in the story. Maybe Harry is under the influence of a dark magic or a powerful and evil entity that makes him lose control and start to destroy Hogwarts. For example, if he was possessed by a powerful dark spirit that wanted to eradicate the symbol of the wizarding world's good, which is Hogwarts.
One possible reason could be extreme self - sacrifice. Maybe he believes that by stabbing himself, he can save his friends or defeat Voldemort in a very unique way. For example, if there is a dark magic spell that can only be broken by his own life force.
One possible reason could be a misunderstanding. Maybe some dark magic was involved that made Harry believe the Weasleys were acting against him or the wizarding world in a way that seemed illegal. For example, if there was a spell that distorted his perception of their actions.
One possible reason could be that Snape suspects Harry of something dangerous or against the school rules. Maybe he thinks Harry is involved in some secret Dark Arts activities and ties him up to prevent him from causing further trouble or escaping while he investigates.
One possible motive could be that Harry is under the influence of a dark spell or curse that makes him lose control of his actions. In some fanfiction scenarios, dark magic can overpower a character's true nature.
It could also be that Harry has a different vision for how to deal with the main threat in the wizarding world. Dumbledore's plans might seem too slow - paced or too conservative to Harry. He may believe that his own more direct and bold approach would be more effective, and thus decides to destroy Dumbledore's plans to implement his own.
One possible motive could be that Harry discovers some long - hidden secrets about Dumbledore that he feels are unethical or immoral. For example, if he finds out that Dumbledore had some ulterior motives in his actions towards Harry throughout the years. Maybe Dumbledore was using Harry for a greater plan that involved a lot of sacrifice on Harry's part without his full knowledge or consent.
In such a fanfiction, Ginny might cheat on Harry with Dean due to a sense of being overshadowed. Harry is often the center of attention in the wizarding world, and Ginny could feel like she's always in his shadow. Dean, on the other hand, might make her feel more special and seen as an individual. Also, it could be that Ginny and Harry have had some unresolved arguments or differences that push Ginny towards Dean.
Maybe in the fanfiction, Molly was being overbearing. Harry might have been frustrated with her constant interference or mothering in a particular situation.
One possible reason could be a misunderstanding. Maybe Harry overheard something wrong about the Weasleys and in a fit of anger, threw them out. For example, if he thought they were plotting against him when in reality it was a big miscommunication.