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/

Following the recent developments that occurred at the end of Season 3, Nezuko and Tanjiro have emerged as an existential threat to Muzan in an unexpected way.

Source: https://www.cbr.com/demon-slayer-yoriichi-tsugikuni-strength-level/

For some of Muzan Kibutsuji's subjects, a demonic metamorphosis is the natural evolution of a life of evil.

Source: https://www.cbr.com/demon-slayer-akaza-backstory-trivia-fun-facts/

Having been granted immortality by the Blue Spider Lily, Muzan sought to replicate the doctor's work with the flower in order to find a way to eliminate his weakness to sunlight.

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

However, Zenitsu's strength comes from his speed, something that even Muzan Could not react to.

Source: https://gamerant.com/demon-slayer-inosuke-vs-zenitsu-whos-stronger/

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

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

It should be noted that when it comes to overall strength and stamina, Inosuke's abilities are far more impressive than those of Zenitsu, as was best seen in the final battle against Muzan where he got up against the Demon King far more times.

Source: https://gamerant.com/demon-slayer-inosuke-vs-zenitsu-whos-stronger/

Kagaya could never beat Muzan or any demon in a fight personally, but he makes up for it with his genuinely inspiring, motivating and empowering leadership style that has kept the Hashira and demon slayer corps together all this time.

Source: https://www.cbr.com/demon-slayer-kagaya-ubuyashiki-vs-muzan-why-winner-leadership/

Muzan expects his demons to keep getting stronger because their survival and ability to consume humans depend on it.

Source: https://gamerant.com/demon-slayer-kimetsu-no-yaiba-demons-that-defied-muzan/

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/

Sanemi managed to survive the battle against Muzan, if just barely.

Source: https://www.cbr.com/demon-slayer-ending-explained-every-hashira-fate/

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/