It seems cruel, but the demon has his own twisted reasoning for harming Tanjiro. Why did Gyutaro break Tanjiro’s fingers?
Gyutaro chooses to break Tanjiro’s fingers over a lack of sibling protection. The demon says Tanjiro’s hands should be used to protect his younger sister, which he fails to do, so Gyutaro breaks them.
The demon previously asks Tanjiro what his relationship is to the person in his box. Once Tanjiro admits that Nezuko is his younger sister, Gyutaro laughs at the failure of protecting her.
Gyutaro chooses to break Tanjiro’s fingers
In chapter #92, the tides of battle between the Demon Slayers and Gyutaro have taken a sinister turn. The demon successfully overpowers the group, breaking them up.
A scornful Tanjiro wakes up to Gyutaro hovering over his head. The demon mocks him, saying the boy is lucky to be alive. According to Gyutaro, everyone else is a goner.
As Inosuke runs away with Daki’s head, Gyutaro unleashes a single thrust through the boar boy’s chest.
Zenitsu is buried under rubble, wriggling around uncomfortably as he clings on to life. Tengen, the Sound Hashira, lays on the ground with no heartbeat. The poison reaches his hurt, and blood gushes out due to a cut-off arm.
Gyutaro calls them all pathetic, but says Tanjiro is the most pathetic of them all. The demon correctly assumes that the demon Tanjiro carries in his box is a relative. Tanjiro admits the demon who can fit in the box is his younger sister.
The demon laughs, saying that Tanjiro is completely failing to protect her. Despite Nezuko being a demon and Tanjiro being a human, Gyutaro explains, it’s still Tanjiro’s responsibility to use his hands to protect her.
As Gyutaro says this, he holds Tanjiro’s fingers and breaks them. The demon calls Tanjiro a worm, saying that his little sister has used up all her strength to protect him.
So that’s the answer. Gyutaro chooses to break Tanjro’s fingers because Tanjiro fails to protect his sister.