It was a fierce battle that ultimately results in his demise, but Rengoku was able to crack a smile after his fight against Akaza. Why did Rengoku smile when he died?
Rengoku was able to smile when he died because he saw a vision of his mother. She says that he did “splendidly,” using his natural-born strength to protect those weaker than him. This sense of pride from his mother causes Rengoku to smile before he passes away.
The Fire Hashira was able to successfully save the lives of all upon the Mugen Train. He also protects Tanjiro, Zenitsu, and Inosuke during the battle against Akaza.
Rengoku’s flashback of his mother
In chapter #64, Rengoku has a flashback of a conversation with his mother. She encourages Rengoku to think hard and deeply consider her question.
She asks if Rengoku knows why he was born stronger than other people. He thinks for a moment, but says he doesn’t know.
Rengoku’s mother explains it’s so that he may help people weaker than him. Those with more natural talent than others, she continues, must use their strength for the good of others. The mother says it is “unforgivable” to wrongly use ones’ strength to hurt others and empower yourself.
“Helping the weak is the duty of those born with strength,” she says. “You must take your responsibility seriously and do your duty, and never, ever, forget that.”
The young Rengoku passionately responds “Got it!” His mother holds him in her arms and says she doesn’t have much time left to live, but she’s happy to have been the mother of such a kind and strong child.
Renogku is able to smile when he died after seeing his mother
In chapter #66, Rengoku speaks his final words to Tanjiro, Zenitsu, and Inosuke. He was pierced through the stomach by an attack from Akaza, an Upper Rank demon. Rengoku didn’t become a demon, refusng Akaza’s offer.
Before Rengoku passes on, he sees a vision of his mother. He asks her if he did well, and if he used his natural-born gifts wisely.
His mother responds “You did splendidly.” In Rengoku’s final moments, he smiles. His mother’s approval of protecting every citizen on the Mugen Train, as well as the younger Demon Slayers, fills Rengoku with pride before he passes on.
So that’s the answer. Rengoku was able to smile when he died because he saw a vision of his mother. She says he used his strength “splendidly” to protect those weaker than him, and the Fire Hashira was able to pass away in peace.