My Hero Academia is a series with a clear message regarding the importance of lending others a helping hand, which is part of what makes Tomura Shigaraki the most misunderstood character in the story. That is because a lot of people tend to associate his descent into evil as the cause of his abusive childhood, but it goes beyond that.
The main reason that Shigaraki became a villain in My Hero Academia is because no one wanted to help him when he was on his own, and then All For One manipulated him to the point he felt that revenge against the world was justified. This doesn't make his actions righteous in any shape or form but rather depicts him as a cautionary tale of what happens if people are not helped.
Disclaimer: This article contains spoilers for the series.
Explaining the nature of Tomura Shigaraki's character in My Hero Academia

It is often misunderstood that Tomura Shigaraki, initially Tenko Shimura, became a villain because of the physical abuse he suffered from his father, Kotaro Shimura. While this situation did play a role in his descent into evil, it was the lack of support from others when his family died and the corrupt influence of All For One that ended up pushing him over the edge.
Ever since the story started by introducing Izuku "Deku" Midoriya and how he was bullied for not having a Quirk, the series has been showing the consequences of not helping those who are in need. Shigaraki is the most extreme result of not getting that support, since he was groomed by All For One to believe he has a right to destroy the world and the people living in it, which is the core of his essence as a villain.
It is not simply the element of a bad childhood, which definitely played a role too, but the fact that he grew resentful of others not wanting to help him and was pushed by a villain to believe he was justified in destroying everything. Furthermore, author Kohei Horikoshi has made this a running theme throughout the story with several characters.
The main theme of the series

As mentioned earlier, My Hero Academia is all about the hardships someone goes through and how the characters react. In that regard, there are examples such as Deku, Shigaraki, Dabi, Spinner, Shoto Todoroki, or Mezou Shoji, who have suffered abuse in some shape or form, but they all reacted in their own unique ways, giving them a lot more personality and their own moral compass.
On the other hand, there are characters such as Katsuki Bakugo and Endeavor, who have caused pain to others and have tried to become better men to help those around them. That is a major theme of the story and is perfectly reflected through Shigaraki, who is someone who simply cannot let go of his pain and corruption, dying with that mentality until the very end.
Final thoughts
Tomura Shigaraki might be the most misunderstood My Hero Academia character because his origin could confuse some people regarding what Horikoshi is trying to convey. However, he is ultimately the result of abuse, lack of care, and corruption, eventually turning into the worst possible outcome when it comes to people who have gone through what he has lived.
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