WARNING: The following contains spoilers for Demon Slayer: Kimetsu no Yaiba Season 2, Episode 14, "Transformation," now streaming on Funimation, Hulu, and Crunchyroll.
While it certainly seemed like Nezuko Kamado was here to save the day in the battle against Daki, her new, more demonic form has turned out to be the worst-case scenario for the poor girl. The entire premise of Demon Slayer: Kimetsu no Yaiba is that Tanjiro Kamado is trying to find a way to turn his sister back into a human. In the meantime, she fights by his side as a special demon who does not devour humans. However, the seventh episode of the "Entertainment District" arc has proven that there is much more to Nezuko's demon hood than anyone ever knew.
When Nezuko debuted her new form, it initially appeared to be a simple powerup that would help her fight Daki, Upper-Rank Six of the Twelve Kizuki. Given the dire circumstances with Tanjiro unconscious and backup miles away, there was no time to look this gift horse in the mouth when Nezuko grew a horn and her power shot through the roof. Daki was even caught off guard when Nezuko's regenerative abilities surpassed those of an Upper-Rank, allowing Nezuko to bludgeon Daki with her repeatedly regenerating leg.
When Season 2, Episode 14, "Transformation," picked up, it was revealed that Nezuko has completely lost herself in her new form, smiling sadistically as she gleefully tormented Daki. Considering all the awful things Daki has done, it's not like she elicited any sympathy from Nezuko's beatdown. That said, Nezuko seemed to be so thoroughly enjoying herself that it was alarming. Whether it was Tanjiro's subconscious or the actual spirit of one of his deceased siblings, even unconscious Tanjiro could tell that Nezuko was about to cross an unforgivable line.
Nezuko's power in this new form was overwhelming, even though Tengen Uzui later showed in the same episode that defeating Daki isn't that impressive. More important than Daki's dismemberment was the fact that Nezuko wounded nearby humans during the fight, and was drawn to their blood. Nezuko had defied her demonic craving for blood in the past, but her control in this form appeared non-existent. She hungrily launched herself at the bleeding woman and almost devoured her. If Tanjiro had not stopped her, she surely would have torn the woman apart, consuming human flesh for the first time.
Even though Tanjiro managed to prevent her from harming anyone, it may already be too late to save Nezuko. After all, Tengen Uzui was one of the Hashira most vehemently opposed to allowing Nezuko to live in the first place, and now he has witnessed her lose control firsthand. The Sound Hashira seemed content to leave her to Tanjiro for the moment, but it might just be because he has other things to address first, such as the demon whose head he cut off but who was not dying. Once the battle in the Entertainment District is over, assuming anyone survives it, Tengen might bring his attention back to the fact that Nezuko attacked a human, even if unsuccessfully.
Now that Nezuko is back in her human form and sound asleep, the danger for her appears over for the time being. Whether or not her fate has already been sealed is yet to be determined. Tengen and Tanjiro also have much bigger problems now that Gyutaro, the true Upper-Rank Six, has appeared. Tanjiro certainly wouldn't want to see his sister in such an awful state again, but Nezuko learning to control her new form might be her only chance at being spared from execution now that her refusal to consume humans is officially debatable.