WARNING: The following contains spoilers for Demon Slayer: Kimetsu no Yaiba Season 2, Episode 18, "No Matter How Many Lives," now streaming on Funimation, Hulu and Crunchyroll.
A large portion of Demon Slayer: Kimetsu no Yaiba's Season 2 finale was dedicated to the backstories of the "Entertainment District" arc's antagonists, Gyutaro and Daki. Two siblings who love each other more than anything and have no other family, the Upper Six demons are dark mirror images of Tanjiro and Nezuko. In the nature vs. nurture debate, Demon Slayer appears to lean toward the nurture side of the argument. The only real difference between the childhoods of Gyutaro and Daki versus Tanjiro and Nezuko is that the Kamado siblings were privileged and loved, while Gyutaro and Daki were hated and alone.
As is the case with most demons in the series, Gyutaro became overwhelmed with memories of his past on the verge of his death. Demons, for the most part, don't remember their human lives until they are about to die. If there's one thing that Demon Slayer does well, it's making its demons sympathetic characters after they've committed countless atrocities and have paid for it with their lives. The series does not engender forgiveness in the villains, but a sense of pity and general sadness surrounds even the most triumphant decapitation of a despicable demon.
Gyutaro's childhood helps shed some light on why he was so fixated on Tengen Uzui's overall awesomeness, as Gyutaro despises anyone who was born beautiful and loved. As it turns out, Gyutaro's disfigured face and scarred skin aren’t a result of his demonism -- he was simply born that way. Daki, whose human name was Ume, was born beautiful and became someone whom Gyutaro took pride in protecting. And much like Tanjiro did with his own family, Gyutaro came home one day to find Ume's charred corpse as retaliation for wounding a customer.
Tanjiro and Gyutaro have a lot in common in that they both love their sisters and will do anything to protect them. Tanjiro, however, did not succumb to his anger as Gyutaro did. In fairness, Gyutaro didn’t strictly choose to become a demon in his grief. A mysterious demon, the former Upper Six, found Gyutaro and Daki and turned them into demons himself. Of course, Gyutaro may have chosen this path regardless, as the first thing he did after finding Ume's body was to kill two people, one of whom had no part in the attack on her.
Tanjiro was fortunate in that he could take vengeance against his family's killer because a demon killed them. Who knows what Tanjiro would have become if it had been people who'd slaughtered his family? Tanjiro himself noted that one wrong step on his journey could have easily resulted in his becoming a demon with his head beneath the blade of a Demon Slayer. Tanjiro hates demons for taking his family away from him, but he does not take joy in ending their lives. Gyutaro, on the other hand, revels in killing anyone who crosses him.
Perhaps the only real difference between Gyutaro and Ume and the Kamado siblings is that Tanjiro and Nezuko know how to love others, while Gyutaro and Ume only know how to love each other. While not an excuse for their actions, it's understandable how Gyutaro and Daki became what they did. Tanjiro and Nezuko are lucky in that they knew what it was to be good, while Gyutaro and Daki always had to be vicious just to survive. Tanjiro and Nezuko and Gyutaro and Daki are all products of circumstance, and their roles in the series could easily have been reversed if their respective childhoods had been even slightly different.
