WARNING: The following contains spoilers for Demon Slayer Season 2 Episode 18, "No Matter How Many Lives," now streaming on Funimation, Crunchyroll and Hulu.
One of Demon Slayer's most alluring features is how nuanced every major villain is. The story makes it clear that demons are not purely evil, mindless monsters. In fact, they retain much of who they were as humans. Even how they are created varies by the situation. Some saw the power in being a demon and took it, while others were nothing more than sick experiments. A few, though, were not swayed by either Muzan's charm or the prospect of immortality.
While Rui was convinced to become a demon when promised a healthier body, and Akaza was most likely lured in by the prospect of staying in his peak fighting form eternally, these ideas are not what swayed Gyutaro and Daki. Their reason was hinted at in Season 1 of Demon Slayer with mansion demon Kyogai, as it contributed to his transformation. Ultimately, how other humans treated Daki and Gyutaro in life led to them becoming the feared demons they are.
Daki and Gyutaro were given a terrible lot in life. Born into their area's poorest community, they were seen more as burdens than anything. Another mouth to feed greatly affects families that are barely surviving as is, and Gyutaro in particular was seen as a curse. His mother tried to kill him both in the womb and after his birth, but failed each time. He survived on whatever small animals he could hunt for food and was ridiculed by onlookers for being an ugly, smelly street rat. Adults would throw rocks at him and look on in disgust.
By comparison, Daki was a treasure. Her beauty made her immediately valuable and she worked the Entertainment District before even turning 13. That value, however, didn't last long. The moment she stepped too far out of line for her employer, Daki became disposable and was punished in one of the most horrific ways imaginable. For assaulting a customer, Daki was bound and burnt alive.
After he finds Daki burnt to a crisp in a hole, somehow still alive, Gyutaro is attacked as well. Two severely injured children roam the streets, yet nobody stops to offer them help -- not even when Gyutaro collapsed and it started to snow. The one creature who does approach them isn't human but a demon, an Upper Moon. He offers them both some of his blood and, while Gyutaro doesn't show any sign of agreement, he also doesn't object.
Gyutaro lets this demon do whatever he wants because he's the only one who so much as noticed them and offered anything at all. He and Daki had to spend their whole lives taking and scrambling for the slightest mercies. All Gyutaro wanted was to live comfortably with his sister, the only person who wasn't completely disgusted by him. Even that simple desire was ruined because of who and where they were. Demons didn't slaughter his only family. They didn't make working as a debt collector his best option. They didn't want his or Daki's deaths. Humans did.
All the Upper Moon did was take advantage of what humans had already done to Daki and Gyutaro. He didn't manipulate them into becoming demons, didn't promise anything except that they could live. After being told he's a monster at every opportunity, Gyutaro decided he might as well become one.
Kibutsuji Muzan is the origin of all demons in Demon Slayer and is responsible for the vast majority of their creation. However, he is not the reason Daki and Gyutaro became monsters. Their fellow humans did that to them and, had there been even the slightest kindness from their own kind, these siblings may not have gone down the path they did.
