That's just a cover. He did defeat his demon lord after she trained him to be able to do so, but rejected killing her outright and instead broke her "demon lord" status and it's in-built compulsion to destroy everything.
Seems like the brother version of a "live the life I wanted to, what you want doesn't matter" type of parent. Like a sperm-donor who forces their son to play school sports because they always sucked and never made the team.
So now that Cool's made it look like Nina's gone back to her normal daily village life she's going to show up next chapter to play like nothing happened, right?
Dragon king: "That would probably kill a dragon king if they touched it. It would at least make you sick."
Daichi: "What? It's a little tough but I feel fine."