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.
It might be a result of guilt. Maybe he did something unforgivable in the story, like accidentally causing the death of a loved one or betraying his friends, and he can't bear the weight of his actions. So, shooting himself could be his way of seeking punishment or escaping the guilt.
Perhaps Percy had lost someone very dear to him because of Tartarus. This could have filled him with so much rage and a thirst for revenge that he decided to kill Tartarus. In fanfics, emotions often drive characters to extreme actions. Percy, being a hero, might have thought that taking out Tartarus was the ultimate way to avenge the wrongs done to him and those he cared about.
One possible plot could be that Percy is overwhelmed by the pressure of his demigod duties. He has constantly faced life - threatening situations, seen friends die, and felt the weight of the prophecy on his shoulders. In this fanfic, he might see shooting himself as a way to end the cycle of pain and danger for himself and those he loves.
One possible motive could be a misunderstanding. Maybe the Hunt has been misinterpreted as a threat to Percy's friends or Camp Half - Blood. So in his attempt to protect his own, he takes extreme action.
One possible motive could be a sense of betrayal. If the gods had broken promises to Percy or his friends too many times, he might feel pushed to the edge. For example, if they failed to protect his loved ones despite his loyalty and service to them in previous adventures.
Well, it's a bit of a gray area. On one hand, fanfic is a form of creative expression. If the author is exploring complex themes like depression, hopelessness, or the toll of heroism, it can be seen as a valid exploration. However, Percy Jackson is a beloved character, especially among younger fans, and such a dark and extreme plot might be disturbing for some.
One possible motive could be miscommunication. In fanfiction, if there's a misunderstanding between them, Percy might lash out and seem like he's bullying Annabeth. For example, if he misinterprets her actions as a betrayal, he could start being mean to her.
One possible motive could be protection. Annabeth might think that there is a threat to Percy that she can only handle by taking him away from the situation. For example, if there's a prophecy that puts Percy in danger and she believes she can keep him safe in a hidden location.
Another motive could be a difference in ideology. If Percy believes in a different way of handling things compared to Artemis, and he feels that her methods are endangering the world or his loved ones, he might decide to betray her in order to stop her from following her current path.
One possible motive could be self - defense. Maybe Ares was about to harm Percy's friends or family, and Percy had no other choice but to fight back and end up killing him.
Perhaps in the fanfiction, Percy is forced into a situation where he has to choose between Thalia's life and the safety of many others. If the situation is presented in such a way that killing Thalia seems like the only option to save a large number of people, he might be pushed to do it. It could also be related to some prophecy that Percy misinterprets. If the prophecy seems to suggest that Thalia will bring great doom, and Percy takes it too literally, he could end up trying to kill her.