Probably my favorite scene in the entire series. Spoilers.
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This is the best example of heroism I have seen in any show or anime. This guy lives in fear and battles fear just so you don't have to. He's the guy on your team, just because you will let him be. This is the guy who will die for you, just because you believe in him. Just to belong somewhere.
The fact that he was swatted like a fly means nothing. The Mumen Rider walked into certain death with a mighty roar. The Sea King could never reach that level of strength. Mumen Rider has something to die for. Something that lives on past his death.
When the crowd started chanting encouragement, his fear melted, he realized he had everything he could have ever wanted. Right there. No matter whether he's killed or not, he will protect everyone with his life. And he will be loved. And that indeed would have been the end of his story without Saitama showing up.
And The Sea King died with his delusion of power stripped, without even a shred of dignity left to his legacy. With a single punch. Good riddance.