Well, it's a complex situation. Shigaraki dies as a result of a combination of factors including the coordinated efforts of the heroes and his own overexertion of his abilities.
Well, Shigaraki met his end in the manga after a series of intense confrontations. His powers were ultimately no match for the combined efforts of the heroes and the circumstances of the final showdown.
In certain fanfictions, Shigaraki's redemption is tied to his past. Maybe he discovers some long - forgotten memories that make him understand his true self and that he doesn't have to be a villain. He then starts to use his power for good instead of destruction.
Shigaraki is not dead in the manga. The storyline has many twists and turns, and his character plays a significant role, so his survival adds to the ongoing drama and suspense.
To write their relationship realistically, you need to consider their personalities. Deku is kind - hearted and determined, while Shigaraki is more cynical and vengeful. So their interactions would be full of tension. For example, if they have an argument, Deku might try to reason with Shigaraki using his ideals, but Shigaraki would likely mock or reject them.