In this fanfiction, Peter might have discovered some dark secret about himself or his world. He locks himself away to protect others from it or to come to terms with it on his own. He could be in a world full of magic or superpowers, and this secret is related to that. He needs time alone to understand how to control it or whether to reveal it.
Maybe it's his sister, Mary. Mary could have some idea about why Peter has locked himself away. She might be torn between respecting his privacy and wanting to help him. She could be bringing him food and leaving little notes outside his locked door, trying to communicate with him.
It could be the village council. They often make decisions for the safety of the village, and if they perceive Naruto as a threat, they might be the ones to lock him away.
Peter could sacrifice himself to stop a deadly virus from spreading. He might be a scientist who discovers that his own body can be used to create a cure, but it will cost him his life. So, he decides to go ahead with it to save the world.
Another theme could be pain and endurance. Stitching oneself is a painful process, so the fanfiction might explore how Peter endures this pain and what it says about his character. It could also show his determination to keep going despite the injury. Usually, in these stories, there's also a sense of secrecy or isolation, as he might be stitching himself up alone, away from others for some reason.
Perhaps there is a misunderstanding. In the fanfiction, other characters might have misinterpreted Peter's actions or words, and this has led to an uncomfortable situation for him. As a result, he chooses to distance himself to avoid further conflict.
Well, if Bilbo locks himself with monster poison in a fanfic, it really depends on the rules of that particular fictional universe. Maybe he has some sort of hidden immunity or a magical item that could save him. However, if not, the poison would start to affect his body. It might first make him feel weak and dizzy, then as it spreads, it could start to shut down his organs one by one. He could experience intense pain and hallucinations as the poison takes over. It's a very dangerous situation for him.
One possible reason could be extreme stress. Peter Parker has a lot on his plate, being Spider - Man and having a normal life. If in the fanfiction he faces a series of really tough situations like failing to save someone important or being hunted by a very powerful enemy, the stress might get to him and lead to self - harm as a way of coping. Another reason could be feelings of isolation. Even though he has friends, he might feel like no one truly understands the dual life he leads, so in the fanfiction this could drive him to such a desperate act.