Before Muzan Kibutsuji found him, he labeled Akaza a demon solely because he was strong enough to kill hundreds of trained fighters with his bare hands.

Source: https://www.cbr.com/demon-slayer-strongest-demon-moons/

Demon King Muzan Kibutsuji has seven hearts and five brains, making him one of the most intelligent characters in the anime series.

Source: https://gamerant.com/demon-slayer-smartest-characters-anime/

During Japan's Heian Period, roughly 1,000 years prior to the Taisho Period, during which the anime and manga are set, Muzan Kibutsuji was declared stillborn after his heart stopped beating multiple times inside his mother's womb.

Source: https://www.cbr.com/demon-slayer-muzan-kibutsuji-origin/

In fact, his skills allowed him to stand head-to-head with Muzan Kibutsuji.

Source: https://www.cbr.com/demon-slayer-blade-color-meanings/

Other than the ultra-villainous Muzan Kibutsuji, the demons were once innocent or at least somewhat sympathetic human beings, even the grotesque hand demon and the arrogant himedere Daki.

Source: https://www.cbr.com/demon-slayer-tokyo-ghoul-villains-lose-humanity/

Protagonist Tanjiro Kamado returned home to find his family slaughtered, and he learned that his true enemy was now the demon king, named Muzan Kibutsuji.

Source: https://www.cbr.com/creepiest-anime-character-introductions/

The strongest demons in are handpicked by Muzan Kibutsuji and gain considerable strength by drinking his blood.

Source: https://www.cbr.com/demon-slayer-strongest-demon-moons/