Perhaps there was a curse or some dark magic at play. A curse that made the hunters see Percy as an enemy when he was actually an ally, leading to his untimely death.
One possible reason could be that hunters have been interfering in Percy's quests. If they show up uninvited and start taking over or causing chaos, Percy would get angry. For example, if they start hunting a monster that Percy has been tracking for a long time.
One possible theme could be the exploration of power dynamics. The hunters may be in a position of power over Percy, and the abuse could show how power can be misused. Another theme might be trauma and its aftermath. Percy would likely experience a great deal of emotional and physical trauma from the abuse, and the fanfiction could delve into how he copes or fails to cope.
Maybe Percy unknowingly breaks some ancient and very strict laws of the Gods. For example, he might interfere with some sacred rituals or places that are off - limits to mortals, even demigods like him. In the fanfiction world, the Gods could be very unforgiving when it comes to such violations and decide to kill him as a result.
One possibility is that there is a prophecy that involves Percy and the hunters in a way that the hunters misinterpret. They believe that by torturing Percy, they can prevent some great disaster. However, Percy is actually an important part of the solution, not the problem. As the story progresses, he could start to communicate with some of the more sympathetic hunters and try to make them see the truth.
Perhaps the hunters always interfere in Percy's adventures. They might show up at the wrong time and disrupt his plans. For instance, Percy was about to rescue someone, and the hunters came and took over, but in a way that put the person in more danger. This would make Percy resent them. Also, if the hunters have some internal politics that end up affecting Percy negatively, like they accuse him of something he didn't do just to protect their own reputation, that could lead to his hatred.
One possible scenario is that the hunters disguise themselves as normal people and approach Gabe's place. When Gabe least expects it, they reveal their true identities and take him by surprise, saving Percy. Another could be that they use their tracking skills to find Percy's location even if Gabe has hidden him well.
It could be because Percy is different. If he comes from a different background or has a unique personality trait, the bullies might target him. In the fanfiction, if Percy is from a different race or has an unusual way of speaking, the bullies might pick on him for not being like them.
One reason could be his regret over his past actions. For example, when he distanced himself from his family to pursue his career at the Ministry. He might realize later how wrong he was and cry out of remorse.
Maybe it's part of a plot to have Percy gain some sort of power or knowledge. In many fictional stories, a character has to go through great pain and suffering before emerging stronger or wiser. The torture in Tartarus could be a way to set up this transformation for Percy.
Perhaps Percy doesn't have the right group of friends to play poker with. If his friends are all into different kinds of activities, like studying magic or fighting monsters, there may not be anyone around to initiate a poker game. So, he just never gets into it.