Alright, I've been following 'My Hero Academia' since the beginning and the whole 'Dark Deku' arc hit me differently than I expected. I think the biggest shift was with All Might. For years their dynamic was this pure, aspirational thing—the fanboy and his idol, the symbol of peace passing the torch. Seeing Deku push him away, seeing All Might literally unable to reach him... it broke the core of their relationship. It wasn't just student and teacher anymore; it was someone who needed saving being pushed away by the person who saved him. That reversal was brutal.
It also completely rewrote his connections with Class 1-A. Before, he was the heart of the class, the guy who'd break his bones for anyone. Suddenly he's a ghost, a solo operator shutting them all out. Bakugo's apology became almost immediately irrelevant because Deku was past the point of accepting help. Uraraka's gentle concern wasn't enough anymore. They had to fight him, physically restrain him, to prove they were his equals and his partners. It turned them from classmates chasing him into heroes who had to save him from himself, which is a much more mature and painful dynamic.
Honestly, the most interesting change was with the villains. He started acting like them—solitary, ragged, operating outside the system. It created this eerie parallel where the line between how a hero and a villain might operate got blurry, which made everyone around him question what being a hero even means if it drives you to that state.