Maybe it's due to the isolation he feels. Even though he has friends, there are aspects of his life that no one else can fully understand. He is the Boy Who Lived, which sets him apart. This sense of being different and alone could lead to such a dark state of mind. For example, after the death of Sirius, he felt extremely alone, and if more such tragedies piled up in the fanfiction, it could push him to this extreme.
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.
Perhaps he is under the influence of dark magic. There could be a spell or a curse that makes him lose his will to live. Or, in the fanfiction world, he might be bullied or persecuted to an extent that he sees no way out but to end his own life.
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.
One possible reason could be his overwhelming guilt. Snape had a complex past, being involved in the Death Eaters and then switching sides. He might have felt that he could never truly be redeemed, especially with all the lives lost during the wizarding war that he had some part in, whether directly or indirectly. This guilt could have driven him to such a desperate act.
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.
One possible reason could be jealousy. Maybe she sees Harry getting too much attention or having relationships with other characters that make her feel left out and envious, which then turns into hatred and a desire to kill. Another reason might be a misunderstanding. Harry could be involved in some secret mission or activity that Ginny misinterprets as something harmful, leading her to think he's a threat.
One possible reason could be the extreme pressure he faces. Merlin has to constantly hide his magic in a world where it's feared and persecuted. This secrecy and the fear of being discovered might have built up to a point where he feels there's no way out. Also, his loyalty to Arthur might play a part. If he feels he has failed Arthur in some way, like not being able to protect him fully from threats, it could lead to such despair.
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 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.
Merlin could use his magic to show Harry visions of a better future. By giving him hope, Harry might change his mind. For example, Merlin could show Harry how his actions in the future would lead to great good.