While the Twelve Kizuki play a pivotal role in the Demon Slayer series, the Lower Rank demons weren't significantly shown, apart from Rui and Enmu. Among the Lower Rank demons was the Lower Rank Four demon, Mukago. She was briefly shown in the series, only getting killed by Muzan shortly after her appearance.
While she didn't show anything major to persuade or add to the narrative of the series, upon witnessing her death, many fans started asking, "Why did Muzan kill Mukago in Demon Slayer?" After reading her mind, Muzan learnt that Mukago was weak and a coward, which infuriated him, leading Muzan to kill Mukago.
Disclaimer: This article contains spoilers from the Demon Slayer anime/manga.
Mukago's death at the hands of Muzan in Demon Slayer, explored
Mukago was featured in the Demon Slayer anime during the Rehabilitation Training arc in episode 26 of the anime and chapter 51 of the manga. She first appeared alongside all the other Lower Rank demons to heed Muzan's summons. The reason for the Lower Rank's gathering was Rui's death to Tanjiro during the Mt. Natagumo arc.
When Muzan appeared under the guise of a woman, confusing others, Mukago tried to apologize to him on everyone else's behalf, but was silenced by Muzan almost immediately. After informing them about Rui's death, Muzan asked them why the Lower Moon demons were so weak even though they were part of the Twelve Kizuki.
As they gathered, Muzan started to read the minds of the lower-ranked demons, an ability that came from him sharing his blood with them. Muzan hardly cared about the other Lower Rank demons, except for Rui, as seen by his anger when he learned about Rui's death.

After Muzan finished off Kamanue, he turned his attention to the rest of the demons. When asked if she was more scared of the Hashira or Muzan, Mukago answered that she would lay everything on the line for Muzan, but was shortly corrected by him that Mukago would rather flee from a Hashira, after reading her mind.
Despite her plea and signs of loyalty, Muzan decided to show no gratitude towards their willingness and decided to kill her because of her readiness to flee if she faced a Hashira. As Muzan brought his monstrous arm over her body, Mukago watched herself getting crushed and killed moments later.
Muzan was so engaged that she decided to kill all the Lower Moon demons, including Mukago. Additionally, when reading her mind, Muzan realized how cowardly she was, despite being a demon of the Twelve Kizuki. This furthered Muzan's rage, leading to Mukago's death.
Final thoughts
Unlike Rui or Enmu, Mukago's character didn't really bring anything noteworthy to the story, or rather, wasn't portrayed to show any potential. The Lower Ranks from the Twelve Kizuki were mostly treated as a filler in the anime and were eliminated as soon as introduced in the series, including the demon in question in this article, the Lower Rank Four demon—Mukago.
While the Lower Rank demons failed to leave any impact on the story, the Upper Rank demons and Muzan were set to fight against the Hashira and the rest of the Demon Slayer Corps. They were teleported into Nakime's domain, marking the beginning of the first battle in the Infinity Castle arc.
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