Often, it's shown through his internal thoughts. The writer might describe Harry having angry and bitter thoughts about his parents when he's alone, like constantly thinking about how they left him to face so many difficulties. For example, he may think 'My parents left me to deal with Voldemort, they didn't care.'
His actions can also display this hatred. He might avoid anything related to his parents. In a fanfic, he could refuse to visit their graves or not want to inherit any of their magical items. This is a more outward sign of his internal hatred.
Often, it's portrayed as a very heart - wrenching moment. Harry may be alone in his dormitory, looking at a picture of his parents and the tears just start to flow. The descriptions might focus on his physical state, like his shoulders shaking and his face contorted in pain.
In some fanfics, Bella might avoid any physical contact that could lead to intimacy. For example, she quickly steps back when someone tries to get too close.
His thoughts are also used to portray his hatred. The author might write from Harry's perspective, and we can see his negative thoughts about people. He could be constantly thinking about how everyone is out for themselves or how no one really understands his pain, which then colors his view of everyone around him.
It could be shown through their interactions. For example, Laurel might give Felicity cold glares or make snide remarks whenever they are in the same room. This is a simple yet effective way to convey her hatred in the fanfic.
He would be overjoyed. It's his parents, after all.
At the start, Harry would be stunned. His whole life has been built on the idea of his parents being heroes. When he realizes they betrayed him, he would be heartbroken. He might go on a journey to find out the real motives behind their actions. Along the way, he could become bitter and vengeful. However, he might also find some inner strength and decide to rise above the betrayal. He could use it as a motivation to break free from the shadows of his parents and create his own path, but not without a great deal of emotional turmoil first.
In Harry Potter fanfiction, parents can be portrayed in various ways. Some are shown as overprotective, like Molly Weasley who is fiercely protective of her children in the original series, and this trait might be exaggerated in fanfiction. Others could be absentee parents, perhaps too busy with their wizarding jobs or studies to be fully present in their kids' lives. For example, in some fanfics, pure - blood parents might be preoccupied with maintaining their family's status in the wizarding world.
He could hate them because he felt they were absent during his difficult childhood. Naruto faced a lot of loneliness and bullying, and if he thought his parents could have prevented it but didn't, that would breed hatred.
In some fanfics, Hermione's abusive parents might be shown as neglectful. They could be constantly criticizing her intelligence or magical abilities. For example, they might not understand her love for books and learning magic, and instead push her towards more'muggle' - appropriate activities. This creates a contrast to her life at Hogwarts where she is celebrated for her brains.
Often, his character is shown as vulnerable yet strong. For example, in some fanfics, during a panic attack, he might be scared and shaken, but still tries to keep it together for the people around him.