Obviously, the Dursleys themselves are the main antagonists. Vernon Dursley, with his short temper and extreme dislike for anything out of the ordinary, could be the driving force behind Harry's death. He's always been shown to be cruel to Harry in the original series. Petunia Dursley, too, could play a major role. Her envy of her sister and her hatred for Harry could lead her to either directly cause his death or be complicit in it. And Dudley, with his spoiled and bullying nature, might also be a key figure in Harry's demise.
In this fanfiction, the Dursleys are the prime antagonists. Vernon is likely to be the main one as he is the head of the household and has the most power. He has a history of being verbally and physically abusive towards Harry. He might make decisions that lead to Harry's death out of his own self - centered and bigoted views. However, Petunia also has her own motives. Her jealousy of Lily and her resentment towards Harry for being a wizard could push her to be part of the plan to kill Harry. Dudley, on the other hand, could be influenced by his parents or act out of his own jealousy of Harry's specialness and be a catalyst for Harry's death.
It could be a new character, perhaps a corrupt wizard in the Ministry of Magic who has a vendetta against the Dursleys for some reason. This wizard might make their time in jail even more difficult by withholding basic rights or subjecting them to unfair treatment. And he might be doing all this for his own gain, like trying to steal the Dursleys' Muggle money or something.
It could be Voldemort. He might have influenced the Dursleys in some way to harm Harry, and Severus has to save Harry from Voldemort's long - reaching plans. For example, Voldemort could have placed a curse on the Dursleys' house that would eventually harm Harry, and Severus discovers it just in time.
In this type of fanfiction, a possible plot could be that the Dursleys' abuse takes a more extreme turn. They might starve Harry to the point of death, as they've always been cruel about his food. Since they've kept him in a cupboard for so long, they could also lock him in there without air or water until he perishes. Also, the Dursleys could spread false rumors about Harry in their neighborhood, which incites the neighbors to turn against Harry. This mob mentality could lead to a situation where Harry is killed. For example, the neighbors might believe that Harry is a dangerous criminal in - training due to the Dursleys' lies and take matters into their own hands.
One possible plot could be that the Dursleys, out of their long - held hatred and envy, finally find a way to use dark magic against Harry. Maybe they make a deal with some dark wizards who provide them with a powerful curse. They manage to cast it on Harry when he's at his weakest moment, perhaps during one of his vacations at Privet Drive.
One possible scenario could be that the Dursleys, out of their long - held hatred and jealousy, manage to poison Harry's food. In the fanfiction world, they might find a way to obtain a powerful and undetectable poison. Another could be that they stage an 'accident' like a fire in his room while he's asleep. They could block his escape routes and let the fire consume him.
The Dursleys could also be antagonists in a way. If Draco comes to their home with good intentions, they might not believe him and be hostile towards him. Their over - protectiveness of Dudley and dislike for anything magical could make them oppose Draco and any interaction he tries to have with them.
They would be devastated. Harry has many friends and allies. People like Hermione and Ron would be heartbroken and would probably seek revenge. The Hogwarts faculty would also be deeply affected. Dumbledore, if he's still alive in the fanfiction, would feel a great sense of guilt for not protecting Harry better. There might be a split in the wizarding community, some believing the Dursleys were acting on their own, others suspecting a darker conspiracy.
The Ministry of Magic would be in a difficult position. On one hand, they would need to keep the wizarding world calm. On the other hand, they would have to deal with the fact that a muggle family killed a famous wizard. There would be debates about how to handle the Dursleys. Some might want to obliviate their memories and send them to Azkaban, while others might think that a more public trial is necessary to show that the wizarding world does not tolerate such actions against one of their own.
Sasuke could turn into the main antagonist if Naruto is killed. Their bond was so strong, and Naruto's death might break Sasuke completely, leading him to go against everything Naruto stood for and become a new threat.
Obviously, Harry Potter is one of the main characters. He is the one who has the power to control the Dursleys. And of course, the Dursleys - Vernon, Petunia and Dudley - are also main characters as they are the ones being controlled by Harry.