Merlin could use his magic to show Harry visions of a better future. By giving him hope, Harry might change his mind. For example, Merlin could show Harry how his actions in the future would lead to great good.
In this 'harry hates himself fanfiction', a plot could involve Harry being framed for a crime he didn't commit. Everyone at Hogwarts turns against him, and he starts to believe that he must be a bad person if so many people think he's guilty. So he begins to hate himself. Also, it could be related to his relationship with Voldemort. If he starts to see similarities between himself and Voldemort in terms of power or ambition, that could drive him to self - hatred. Another idea is that he fails an important mission and blames himself so much that it turns into self - hatred.
One possible reason could be the extreme pressure he faces. Merlin has to constantly hide his magic in a world where it's feared and persecuted. This secrecy and the fear of being discovered might have built up to a point where he feels there's no way out. Also, his loyalty to Arthur might play a part. If he feels he has failed Arthur in some way, like not being able to protect him fully from threats, it could lead to such despair.
Hermione could be a main character. She might try to figure out why Harry hates himself and be the one to pull him out of that dark place. Ron could also be there, maybe at first not understanding Harry's self - hatred but later becoming a source of support. Dumbledore could be another key character, as he often has insights into the students' minds and could play a role in helping Harry deal with his self - hatred.
One common theme is his past actions haunting him. Bakugou has been aggressive and mean in the past, and in these fanfics, he often regrets those actions which leads to self - hatred. Another theme is his comparison to others, especially Deku. Feeling like he's not as good as Deku can make him hate himself. And the theme of isolation also comes up. He might feel alone with his self - hatred, thinking no one would understand or accept him if they knew how he really felt about himself.
Gwen would likely become more determined and strong - willed. Merlin's sacrifice would be a huge emotional blow to her. It might make her more resolute in fighting for the good he believed in, perhaps becoming a more fearless leader in Camelot. She may also carry a sense of guilt and a need to prove that his sacrifice was not in vain.
One good 'bakugou hates himself fanfic' could be 'Internal Struggles'. It delves deep into Bakugou's self - loathing and how he copes with it in the face of his peers' success. Another might be 'The Hidden Self - Hate' which shows his internal monologues where he berates himself for past actions. There's also 'Breaking the Self - Hatred' which is about his journey to overcome this self - hatred with the help of his friends.
Arthur would likely be shocked and feel guilty. He might think that he has failed Merlin in some way, not realizing the full extent of Merlin's inner turmoil. He would probably become more protective and try to find out what made Merlin feel this way.
One possible reason could be extreme loneliness. In the Merlin universe, if Harry felt he didn't belong or was constantly ostracized, it might lead him to such a dark place. Another reason could be a sense of powerlessness. Maybe he was faced with a seemingly unbeatable foe and thought death was the only escape.
One possible scenario could be that Gwen is in danger from a powerful dark magic user. Merlin steps in to protect her, knowing that the only way to defeat the enemy is to sacrifice his own life force or magic. Another could be that Gwen is wrongly accused of a crime she didn't commit, and Merlin sacrifices himself to prove her innocence and save her from execution.