Percy is likely a character within the context of this fanfic. Without more information, it's hard to say exactly who he is, but he's probably someone with the ability to take the power of things he kills.
Well, it could be that in this fanfic, Percy has the unique ability to absorb the powers of the beings he kills. This might make him extremely powerful as he accumulates various abilities over time.
The power structure would be completely disrupted. There would be no more divine interference in mortal affairs, which could lead to chaos at first as there are no gods to control natural elements like the sea or the sky. But it could also lead to new opportunities for mortals to take control of their own destinies.
There could be some unexpected allies as well. Maybe some minor gods or goddesses who are tired of being overshadowed by the major gods. They might see Percy's plan as a way to change the power structure in the divine world. These minor deities could provide insider knowledge about the major gods' habits, lairs, and weaknesses, which would be extremely valuable for Percy in his mission.
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 the Gods' fanfic, there is a new and extremely powerful enemy that the gods are unable or unwilling to fight effectively. Percy sees this as a sign of their incompetence or callousness. He could gain some new, super - powerful abilities or weapons that give him the confidence to challenge the gods. He might start by targeting the more tyrannical gods first, like Zeus, who has a history of being harsh and unfair.
The power to manipulate elements like water or earth could also be really exciting. Since Percy already has some connection to water, adding earth - manipulating powers would make him more versatile. He could create barriers of earth, or use water and earth together in new and creative ways. It would change the way he approaches battles and problem - solving in the fanfic.
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.
In this fanfic, Percy is the central figure. However, there might be antagonists too. These could be other powerful individuals or groups who don't want Percy to take over the world. They would try to stop him at every turn, creating a lot of conflict. There could also be some characters who are neutral, just trying to survive in the changing world Percy is creating.