The two end up becoming friendly later in the plot, so why did Kanao attack Nezuko?
Kanao chooses to attack Nezuko because she’s clearing out demons. She’s making a safe area for Corps helpers to transport the injured to the Butterfly Mansion.
She does not understand the special circumstances around Nezuko. Kanao attacks her for the sake of being a demon and for no other reason.
Kanao chooses to attack Nezuko
Chapter #44 begins with the Demon Slayer Corps cleaning up after the battle against the Spider Demon. The Insect Hashira, Shinobu, and her apprentice, Kanao, are on the scene.
Kanao informs the Corps helpers that all wounded should be taken to the Butterfly Mansion. She says she’ll take care of any remaining demons nearby so the helpers can work without fear.
We then see the Water Hashira, Giyu, protecting Tanjiro and Nezuko. They are confronted by Shinobu, who wants to attack Nezuko.
Shinobu says she’ll use a non-painful poison against Nezuko. Giyu tells Tanjiro to gather his strength and flee with his sister as fast as he can. The other Hashira do not understand Tanjiro’s special situation with Nezuko, so it would be best for them to run while Giyu cools Shinobu off.
As Tanjiro and Nezuko run away, they are attacked by Kanao. She pins Tanjiro down to the ground, giving Nezuko enough time to run by herself.
As Nezuko runs, Kanao has a sense of confusion. She notices that Nezuko is not attacking and only trying to flee. “Don’t think about it,” Kanao says to herself. “Just kill demons, like I was taught!”
Before the situation with Shinobu and Giyu comes to a head, a crow intervenes. The crow says that Tanjiro and his demon-sister, Nezuko, are to be captured and returned to the Demon Slayer Headquarters.
Kanao stops attacking Nezuko once she hears the girl’s description. The crow says that Nezuko has a bamboo gag in her mouth.
So that’s the answer. Kanao chooses to attack Nezuko because she’s clearing out demons. She’s trying to make a safe area for Corps helpers to move out the injured to the Butterfly Mansion.