One possible reason could be extreme despair. Maybe he faced some overwhelming challenges or betrayals that made him feel there was no way out. For example, if he lost all his friends and felt completely alone in the world of the fanfic.
Perhaps in the 'percy kills himself fanfic', there is a situation where he is being hunted or persecuted constantly. He may have reached a point where he couldn't take the constant fear and pressure anymore and chose to end his own life as a way to escape.
One possible reason could be extreme despair. Maybe in that fanfic's alternate universe, Harry has lost all his friends and loved ones in a very tragic way, like a second wizarding war that was far more brutal and unforgiving. He might see no way out of the emotional pain and thus choose to end his own life.
In such a fanfic, one possible reason could be extreme depression. Maybe he faced overwhelming losses like the death of his closest friends and felt he couldn't go on. Another reason might be that he was under some kind of dark magic influence that made him lose his will to live.
One possible reason could be extreme despair. Maybe in the fanfiction's universe, Percy has faced a series of unbeatable challenges like losing all his loved ones and feeling completely alone. Another reason might be a sense of guilt. If he made a huge mistake that led to disastrous consequences for others, he could be so consumed by guilt that he sees no other way out. Also, if he was betrayed by those he trusted deeply, it could shatter his world view and push him to such a tragic end.
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.
Well, in such a fanfiction, it could be due to a combination of factors. His isolation in Grimmauld Place might have taken a huge toll on his mental state. He might have felt like a burden to his friends, especially Harry. And the lack of any real future prospects, given his fugitive status and the ongoing battle against Voldemort, could have pushed him over the edge into taking his own life.
One possible reason could be jealousy among his peers. Maybe someone was envious of Percy's skills or popularity and spread false rumors about him, leading to his betrayal. For example, if Percy had a special power or was favored by a certain group, others might feel left out and decide to turn against him.
Some writers might do it for shock value. They want to create a very different and extreme take on the character to get a strong reaction from readers.
It could be that Percy's sense of honor led him to sacrifice himself for Zoe. If he had made a promise to protect her, he would uphold it no matter what. Also, Zoe might have been the key to solving a major mystery or threat. Percy, realizing this, chose to sacrifice himself so that Zoe could continue and complete whatever task was necessary. Additionally, his love for his friends and the greater community of demigods might have made him believe that Zoe's survival was more important than his own.
It could be because of a dark power corrupting Sasuke. Maybe an ancient evil force takes over him and makes him see Sakura as an enemy. In some fanfics, this kind of external influence can completely change a character's actions. Also, it might be related to Sasuke's pursuit of power. If he believes Sakura is standing in the way of his ultimate power - seeking goal, he could be driven to kill her.
One possible motive could be extreme depression. If Percy has been through a lot of hardships in the story, like being ostracized or constantly failing in his goals, it could lead him to this drastic step.