The reason Muzan disbanded the lower moons runs deeper than just his anger towards their "weakness"; he was angry, perhaps even sad, that Rui died. Hence, Muzan killed the other lower moons, in part, out of despair. Another reason was Muzan's own vanity.
Muzan's threat are the Hashiras. Instead of mitigating Hashiras one after another, the clever way is to prevent them from forming in the first place. You go after the nest, not the pest. Considering the requirements to become a Hashira, it's in Muzan's best interest to eliminate the Lower Moons.
Muzan then decided Enmu was worth keeping alive so he can attack the demon slayers and gave him more of his blood making him stronger, promising him more blood if Enmu can kill Tanjiro or the Hashira. Enmu's body could hardly process so much new strength from his upgrade and he almost died.
Firstly, he was afraid that the demons would turn on him, especially if they banded together. So he wouldn't want them to have too much power either individually or as a group. Secondly, most demons bodies were not able to handle the power.
1 Muzan Regretted Killing His Doctor
He shows his most human side to fans, wherein he admits to regretting murdering his doctor. He comments that had he been patient, the doctor would have cured him, and he would never have turned into a demon.
The Wind Hashira has one of the saddest backstories in Demon Slayer. His mother had turned into a demon and had killed five of his siblings. He was forced to kill his own mother in an attempt to save his only remaining brother, Genya.
Even after becoming a powerful demon, Muzan considered himself imperfect and wanted to somehow get rid of his weakness of sunlight.
Akaza was also one of Muzan's favorite demons out of all his subordinates. The reason behind this is Akaza's extreme loyalty to his master. The Upper Rank Three demon was always sent away on special missions, something the Upper Rank 1 and 2 were often denied of.
The curse prevents Muzan's name from being spoken aloud, and any who do will meet a horrible demise. One demon, Susamaru, faced this reality when battling Tamayo. Susamaru accidentally shot Muzan's name aloud, attempting to defend his honor with her mind clouded.
The only reason why Muzan hates Doma is because of his insincerity and lack of emotions. This might sound odd, expecting emotions from a demon, but even a demon has to retain such tendencies to keep things in order.
Tanjiro, interestingly, would already be a Hashira if he'd been ranked at Kinoe when he killed Enmu, Lower-Rank One of the Twelve Kizuki, during the "Mugen Train" arc. Unfortunately, Tanjiro, as well as Inosuke and Zenitsu, are only Kanoe -- not even halfway to the top of the Corps' ranking system.
As the main threat occupying the position of Upper Moon Six, Gyutaro's Blood Demon Art is tremendously powerful and extremely dangerous. With it, he can perform various attacks using blood from different parts of his body, and project them from his sickles as well.
Mitsuri puts up a strong fight against the Demon King and finally recovers from her loss of strength. Finally, as Muzan was about to land a critical blow on Mitsuri, Tanjiro stabs Muzan with a broken Nichirin blade in the head, and at the same time, Infinity Castle collapses.
Fortunately, Zenitsu was able to single handedly beat the Upper Moon 6. Not only did he defeat the demon, he also created a seventh form called the Flaming Thunder God. These are the demons who have been given the title of Upper Moon 6 in Demon Slayer.
Yoriichi Tsugikuni was the first Demon Slayer and also the one who invented Breathing Techniques in the first place. Yoriichi was born with the Demon Slayer mark and prowess in swordsmanship. His powers were beyond comparison as he defeated Muzan once and had him on the verge of death until he managed to escape.
It is revealed that Muzan's true fear of dying and immense desire to live and survive was due to him being born "dead," where his heart would constantly stop in his mother's womb and was assumed stillborn until he fought to breathe before his cremation.
Akaza. Because of his desire to become stronger and Doma's strength allowing him to surpass himself to become Upper Two, Akaza harboured an intense hatred of Doma.
Haruka Shiraishi: Muzan's Daughter.
It is revealed that Tamayo's drug had an aging potion mixed in along with the Demon antidote. Due to this, Muzan has aged 9,000 years within the span of three hours.
Yoriichi Tsugikuni, the first swordsmen to ever use Breathing Styles, once nearly killed Muzan with Sun-Breathing, and Muzan has been both physically and psychologically scarred ever since.
14) Muzan Kibutsuji (over 1000)
Being a demon, Muzan is capable of living for many, many years. However, unlike other demons that have aged over hundreds of years. Muzan is well over 1000 years old.
After he neutralizes Tamayo's poison and emerges from his flesh cocoon, Muzan's hair grows in length and turns white due to Shinobu's poison accelerating his age. He also grows numerous mouths all over his limbs, each one lined with a set of incredibly sharp fangs.
Muzan's Blood Demon Art is known as “Biokinesis.” It allows him to alter his body and shapeshift at will. Although all demons have this innate ability, Muzan elevates it by modifying his flesh and organs. Biokinesis is also the only reason Muzan has been able to hide from the Demon Slayer Corps for 1000 years.
Muzan Kibutsuji (鬼舞辻無惨) is the man responsible for transforming Nezuko Kamado into a Demon and the main antagonist of Demon Slayer (Kimetsu no Yaiba). Tanjiro could identify him instantly, based on his demon smell. He was the first human to become a Demon over a thousand years ago.