The Mc/hero kills the demon lord but hears a noise from the other room and turns out to be the demon lord's daughter so he puts on armor and pretends to be a demon general or something like that and decides to raise the demon lord's daughter
I believe its "Uchi no Maou ga Tenshi de tsurai" where the story roughly is: the MC/Hero slained the Demon Lord (before the Demon Lord died, she asked the hero to take care of her daughter, which the MC later willing to do it just to fulfill a mother's death wish), in which he wears the dark knight armor to conceal his identity and take care of the Demon Lord daughter
Maybe your talking about Aesthetica of a Rogue Hero
"The story follows a world where several young men and women regularly find themselves spirited away to a world of sword and magic called Alayzard (アレイザード, Areizādo). Those who survive and return are typically gifted with magic abilities. An international training organization named Babel protects these young returnees and trains them in the use of their power. A wayward hero named Akatsuki has come back to his world, along with a beautiful girl called Miu who is the daughter of the Last Dark Lord, whom he defeated in battle."
Also i thought about Kuro no Shoukanshi or "Black Summoner" : The main character doesn't kill the demon lord but his vassal who was protecting the demon lord's daughter and then takes care of her.