Well, it could be that Hercules has done something to Percy's friends or family. For example, Hercules might have accidentally injured one of Percy's close friends during a heroic adventure. Percy, being very protective, then decides to take matters into his own hands and beats up Hercules. Also, there could be a power struggle between the two, with Percy feeling that Hercules is becoming too powerful and a threat to his own world or community.
It could be Percy himself if he's acting unjustly. Even though he's beating up Hercules, if his reasons are not noble, like if he's just being full of hubris, then he would be the antagonist.
One possible plotline could be that Harry discovers Snape has been secretly working against him in a way that endangers his friends, and in a fit of rage, he attacks Snape. For example, if Snape was found to be involved in a plot to harm Hermione or Ron.
One possible plotline could be that Harry discovers a new dark plot by pureblood supremacists to further marginalize non - pureblood wizards. He decides to take matters into his own hands and uses his magic to fight them off. Another could be that he is pushed to the limit by the constant discrimination from purebloods and finally snaps, leading to a showdown.
In this fanfiction, perhaps there is a misunderstanding between Luke and Percy. Luke might think Percy has betrayed him in some way, like siding with the wrong people in the context of their fictional world, and thus resorts to violence by beating him up.
Perhaps in the fanfiction, Percy is under some sort of dark influence. This could be from a new, unknown magical force. This influence makes him see Jason as a threat to his friends or himself, so he attacks Jason. Jason, on the other hand, is initially confused but then tries to fight back to defend himself while also trying to figure out what's wrong with Percy.
One possible plotline could be that Percy has been betrayed by his closest friend. Maybe this friend revealed a big secret of Percy's, and now Percy feels isolated and heartbroken. He might start to question his relationships and his place in the world.
One possible plotline could be Percy's friends trying to break him out. They might have to overcome various obstacles like guards, traps, and security systems within the prison.
One possible motive could be a misunderstanding. Maybe Hercules is framed for something really bad, and Percy, believing the false information, decides to take action. For example, if there's a plot where Hercules is made to seem like he's about to destroy a city full of innocent people, Percy might feel it's his duty to stop him, even if it means killing him.
Percy could be the instigator if he has a moment of rage or is misinformed. His impulsive nature might lead him to start the fight without fully understanding the situation.
Well, perhaps in the fanfiction, Annabeth betrays Percy's trust in a relationship way. She could start seeing someone else and use Percy's secrets against him to make herself look better in front of the new person. Or she might side with a group that Percy is against, believing their lies and false promises over Percy's warnings. This could lead to Percy feeling completely alone and having to find new allies.