So after Rui's death, why did Muzan kill the lower demon moons, with the exception of Enmu? Muzan gave Enmu some more of his blood which caused him to become even more powerful.
However, I can't help but feel that Muzan killing the lower demon moons was a bad writing decision. So are there any other reasons for him to kill the lower moons?
Muzan is a very cold individual, the lower moons consistently disappointed him, and Enmu showed no fear while Muzan killed the other lower moons, which impressed him.
The wiki does a very good job of explaining Muzan's personality, and how he feels about humans, and his subordinates:
Muzan is cold-hearted, ruthless, intimidating, and exceptionally intelligent. He sees no value in any humans he has encountered and shows little value in his own subordinates. Rarely ever visually upset or happy, he appears completely monotone and detached on most occasions, even when he callously murders his own subordinates.
He tends to be very short-tempered too; he once killed two men and their female companion for simply being rude:
He brutally killed a pair of drunk men simply for being rude to him, and killed their female companion when she had done nothing, all while proclaiming his own perfection and superiority over them, displaying his pettiness as he won't settle for any mockery and will do anything he pleases until the humiliation is remedied.
Killing the lower moons helps to solidify just how cold and heartless Muzan is. When he killed the lower moons, he did so because he had become frustrated by their constant failures, as he began killing them, two, three, four, and six all showed great weakness in the eyes of Muzan by begging for blood, fleeing, denying his opinion, and attempting to flatter him:
After the demise of Kyogai and later Rui, the Lower Rank Five, Muzan was frustrated by the Lower Ranks constant failures. He called the Lower Rank One, Two, Three, Four and the current Six to a meeting, rebuking them and believing he no longer needed the Lower Ranks. As a result, Muzan started to kill the Two, Three, Four and Six one by one, as they respectively were begging for Muzan's blood, trying to flee, trying to deny Muzan's opinion and trying to think about flattering his master.
During this entire event, Enmu showed no fear and demonstrated a more sadistic demeanor which impressed Muzan, subsequently saving his life:
Enmu, the Lower Rank One, was the only Lower Rank left. However, unlike others, Enmu showed no fear over the brink of death and even nonchalantly accepting it for pure excitement. Enmu's sadistic streak impressed Muzan, and he sent Enmu on a mission.
Feel free to do some further reading on the official wiki, and on the villains wiki, which are the sources that support my answer.
Emphasis mine.
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. Without the Lower Moons, now the candidate's only practical choice is to take the longer route of establishing 50 kills, which is significantly difficult than killing a Lower Moon. The strength of candidates may increase, but no matter how strong a candidate is, he/she won't be able to hit any jackpot of multiple Demons, since Muzan implemented a prevention against unionization. Thus, by eliminating the Lower Moons, the benefit of preventing potential Hashiras outweigh the lack of headcount.
Bi-weekly published manga is all about numbers just like any other media. The author and editor must attract as many readers possible to improve sales. Unpopular manga will be terminated and a new manga will appear. Within each manga, unpopular elements are subject to termination, including characters. If one of the Lower Moons had a charismatic character, they would've existed longer.
