The flashback isn’t an explanation of why he’s messed up, it’s symbolic of him admitting he has a problem. He never once apologized for hitting anyone in the original and the flashback promise is sort of his epiphany. Essentially the “i’ll get strong someday” is meant to imply that he’ll mature and pull out all the weeds in his head. It’s also sort of a callback to the first chapter of the original where he was working out and Miya commented how he was surprisingly strong.
The original was categorized as a romance manga so I imagine he was gonna end up with negunegu.
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