As My Hero Academia fans must have observed, while the fanbase often hotly debates Toga and Dabi's past and consequences, not many people have a lot to say when it comes to how Shigaraki Tomura ended up.
Fans often debate how Toga and Dabi could have ended up in different situations had someone intervened in their past. With some guidance at an early stage, both characters could have completely avoided their villainous side.
However, the same does not seem to apply to Shigaraki Tomura as most fans remain convinced that he was fated to end up in the position he did. His only "what if" scenario might be Deku's story in My Hero Academia.
Disclaimer: This article contains spoilers from My Hero Academia manga.
Shigaraki's only "What if" in My Hero Academia might be Deku's story

Unlike Himiko Toga and Toya Todoroki, Tenko Shimura was fated to become a villain from the moment Nana Shimura left her son to fight All For One. After being abandoned by his mother, Kotaro Shimura started harboring hatred towards Nana Shimura. However, instead of getting a grip on his feelings, Kotaro projected the same toward his son Tenko whenever he expressed his desire to become a hero. So, realistically, Tenko was an abused child from quite the get-go.
Amidst this, All For One granted Tenko the very destructive Decay Quirk using which he mistakenly massacred his entire family and destroyed his house. With no place to go, Tenko Shimura wandered the streets hoping for someone to help him. Unfortunately, given his appearance, society abandoned him, believing that some hero would rescue him.

Unfortunately, no one came to his rescue, pushing him further down the rabbit hole, and causing him to antagonize the hero society. Not long after, the "Demon Lord" All For One himself arrived and gave him a helping hand. By that point, All For One was Tenko's "hero," thus, there was no saving him from his fate as a villain.
The only possibility for Tenko to have lived a good life might have been for some hero to rescue him when he was alone in the streets. Unfortunately, even if such a development had taken place, considering All For One's artillery of Quirks, in most cases, the villain could have easily taken Tenko away from the hero and raised him how he saw fit.

This is why Tomura Shigaraki's only good "what if" scenario in My Hero Academia might have been for him to meet All Might somehow and become the next One For All user. Such development was essentially Deku's story that saw him turn into a hero despite being quirkless at the start.
In comparison, Toga and Dabi could have avoided their fates even later down the line as no one targeted them to become villains. Realistically, had the two characters been pushed into situations that forced them to reflect on their actions, they could have gradually avoided their endgame fate.
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