A new, original villain could also be the main antagonist. This villain could be someone from Percy's past who has a grudge against him and wants to use the Avengers as a way to get revenge. By making Percy betray the Avengers, this villain can not only harm Percy but also cause chaos among the powerful superhero group.
It could be a jealous rival. This rival might have always been envious of Percy's popularity and skills. So, they manipulate Percy's friends into betraying him to get Percy out of the way and take over his position.
It could be Ares. He might see the Twilight vampires as a new source of power or chaos. He manipulates Percy into believing that the Twilight characters are evil and need to be stopped. So Ares becomes the main antagonist as he's the one driving Percy to betray the Twilight group.
It could be an ancient evil god who manipulates Percy into betraying the son of chaos. This god might have a long - standing grudge against the son of chaos or his lineage and sees Percy as a useful pawn.
It could be a rogue member of the Hunt. Maybe someone who has a personal vendetta against Percy for some past event or just out of jealousy. This character might be secretly plotting against Percy within the Hunt and is the one who orchestrates the betrayal.
It could be a jealous demigod. Maybe someone who has always been in Percy's shadow and decides to frame him for betraying the gods in order to get rid of him and gain more power and recognition among the demigod community.
Iron Man would be angry. He'd probably use his technology to try and track Percy down. He might also start to review all the security protocols to make sure no other traitors could do the same. Thor would be more of the 'let's fight him head - on' type, but the other Avengers would have to hold him back until they have a proper strategy.
It could be the gods. They often have their own agendas and might see the son of chaos as a threat to their power. So they might manipulate Percy into betraying the son of chaos.
It could be a powerful sorcerer who wants to steal Percy's powers. Since Percy might have some unique abilities, this sorcerer sees her as a key to gaining more power for himself. He could be creating chaos in the city to draw Percy out and then try to defeat the Avengers when they come to her aid.
It could be another powerful god or goddess who wants to use Percy's betrayal of Tartarus for their own gain. Maybe a god of chaos who sees this as an opportunity to create more disorder in the world. They might manipulate Percy into the betrayal and then turn on him later.
If Percy Jackson were in an Avengers betrayed fanfiction, it might start with a misunderstanding. The Avengers could mistake Percy's actions for something malicious. Percy, being used to being misunderstood in his own world, would initially try to explain. But as the situation escalates, he realizes he has to use his powers not just to defend himself but also to expose the real villains who are manipulating the Avengers against him. This could lead to some really cool fight scenes where Percy's demigod skills are pitted against the high - tech weapons and superpowers of the Avengers.