While One Punch Man fans know Saitama as a fun character with the power to defeat any opponent with a single punch, there was one moment in the manga where he showed his weakness. Surprisingly, this incident occurred during a fight with an unexpected opponent.
As fans must remember, during Saitama's fight against Garou, after the monster tossed Genos's core at him, the Caped Baldy stopped caring about anything, including Earth. While it is justified for Saitama to become mad at Garou for killing Genos, the fact that he stopped caring about Earth does not sit well with his persona as a hero.
Disclaimer: This article contains spoilers from the One Punch Man manga.
Saitama's swift change in emotions is his biggest weakness in One Punch Man

As fans must remember, Saitama does not care about anything, including Earth, which was the biggest character betrayal in the series. The incident effectively implied that Saitama did not care much about others, to the point that one of his friends' deaths was enough to make him not care about the planet he lives on and everyone in it.
This development seemed inwardly weak because the incident contradicted the series' beginning, where Saitama risked his life to save a kid. At the time, he had no powers, yet he was willing to risk everything for a kid he did not know. Meanwhile, when he was furious at Garou and possessed his ungodly powers, he did not care if he mistakenly harmed all children on Earth.

The thing that makes even less sense is what Saitama did later. After being furious at Garou for killing Genos and going after him without a care, he shockingly got over his anger mid-fight. With that, his serious expression changed into his oval-shaped cartoonish expression.
This development made Saitama's emotions feel hollow. If Saitama was going to get over his pain mid-fight, there was no point in him acting the way he did. This effectively meant that Saitama had weak control over his emotions. So, had he not reverted to his careless persona, he could have destroyed Earth and the surrounding celestial bodies on a whim.

Moreover, considering Saitama's strength, there was a possibility that he could have taken down Garou much earlier if he wanted to. However, his failure to do that made him look like a horrendous hero who extended the fight, hoping to have fun, which aligned with his original goal of becoming a hero.
What made even less sense was how the entire arc concluded. While Saitama was in anguish due to what happened with Genos, none of that mattered in the end. When Saitama went back in time, he erased everything that happened along the way. Therefore, even though his anguish could have developed his character, the incident was erased. So, had it not been for Garou teaching Saitama his technique, the aftermath of the battle could have been far different.
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