When technology develops to this day: humans have achieved interstellar immigration, developed intelligent optical brains, and created machine armor.
So... The two professions with the longest history on the ancient earth have gradually fallen into decline.
The killer degenerates from "legendary" to "common" to "junk".
This is the story of an Earth-style killer who inherited his father's legacy, and a commoner kid who likes to make money and add bank balances, Moji Moji, gradually develops!