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.
Well, perhaps in that fanfiction, there was a prophecy that was misinterpreted. Harry thought Snape was the key to a great evil about to befall the wizarding world. Also, it could be that someone Harry deeply cared about was seemingly killed by Snape's actions (even if it was a false perception), and his anger led him to this extreme.
If he stabs himself in a non - vital area, he might survive but be severely weakened. This could lead to him having to rely on his friends more for protection and support, changing the dynamics of their relationships.
One possible reason could be heartbreak. If Harry was deeply in love with Hermione and she chose someone else, like Ron, the pain of rejection could be too much for him to handle. Another reason might be if Hermione betrayed Harry in some way, like revealing his secrets to the enemy. This sense of betrayal could shatter his trust and push him to the edge.
One possible reason could be to hide his true identity. In the world of fanfiction, there might be threats or situations where he doesn't want to be easily recognized as Harry Potter. Maybe he wants to start anew, away from the fame and the constant danger associated with his name.
There might be some extremely dark and non - canon fanfictions out there with such a disturbing plot. However, this goes against the general spirit of the Harry Potter series. In the official story, Harry is a hero who overcomes great challenges. In a fanfiction with this idea, perhaps it could be due to extreme despair from losing too many loved ones or feeling an overwhelming burden of his destiny that he snaps and takes his own life. But again, this is a very unorthodox and sad take on the character.
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.
Well, it could be that he has lost all hope. In the fanfiction world, if the situation becomes so dire that he sees no way out for himself or the people he loves. For example, if Voldemort manages to gain complete control and Harry Potter thinks his sacrifice by taking his own life might be the ultimate solution to break Voldemort's power. He might also be suffering from post - traumatic stress after all the battles and losses, leading him to this extreme decision.
In the fanfiction, Harry Potter might live by himself because he is tired of the spotlight. After being in the public eye for so long, he desires a private life. He may also feel that living alone allows him to better understand himself and his powers. It could be that he has a prophecy or a personal quest that requires solitude. For example, he might be trying to master a very difficult and dangerous spell that he doesn't want anyone else to be in danger from if something goes wrong. Also, he could be exploring some dark corners of magic that the wizarding world might not approve of, and living alone gives him the freedom to do so without judgment.
One possible motive could be that the 'mountain' in the fanfiction represents a great evil or obstacle that is threatening the wizarding world. Harry, being the hero, feels it's his duty to eliminate it. For example, if the mountain is a lair for dark creatures that are attacking Hogwarts students.
One possible reason could be the overwhelming burden of his destiny. Harry has faced so many challenges and losses throughout his life. The death of his parents, the constant threat of Voldemort, and the weight of saving the wizarding world might have made him feel so tired and hopeless that he contemplates taking his own life.