In the story of Demon Slayer, demons are supernatural creatures of the night who consume human flesh to survive. They are weak to sunlight, making them a variation of vampires. Like European vampires, these demons view themselves as superior to humans and live on the fringes of society -- but they may try to blend in on occasion.
Monsters often do this so they can scout out and stalk their prey before striking, a wolf in sheep's clothing. Even Demon Slayer's main antagonist Muzan Kibutsuji has been known to do that. Protagonist Tanjiro Kamado met Muzan's false family one night in Tokyo's busy streets, aghast that the human woman and child were so cozy with their worst enemy. But there's a chance that Muzan really did care for them, despite being their ultimate predator.
Why Demon Slayer's Muzan Might Care For His Human Family
Tanjiro couldn't tell for sure what Muzan's false wife and daughter thought of him, or even if the two humans were related. One possibility is that Muzan met and married a single human mother while passing himself off as human. He also may have separately recruited the wife and daughter into his own found family, making him a sort of twisted version of Spy x Family's Loid Forger. For reasons that aren't entirely clear, Muzan needs to integrate into Tokyo's human society, and his convenient found family is the perfect camouflage. In fact, they might be more than that in his eyes.
At the very least, Muzan acted warm and caring to his found family, and the wife was clearly fond of her demon "husband." Even if Muzan views humans as cattle, a small part of him might have made an exception for this woman and child, viewing them as precious novelties and not just another meal. It's easy for the good guy to do that, but even villains can take this route. In Fullmetal Alchemist: Brotherhood, for example, the human-turned-homunculus Wrath felt that way about his human wife, who had no inkling that her politician husband was a lab-built monster. She loved him, and shockingly enough, Wrath was quite fond of her in return, even if it wasn't true love on his part.
As Father's creation with a role to play, Wrath had never been in control of his own life. But he was free to choose any human woman to be his wife and genuinely enjoyed doing so. Wrath mainly liked the idea of creating his own family rather than loving the people involved, but even so, his wife Mrs. Bradley had been a bright spot in his grim life. It's not impossible that Demon Slayer's Muzan feels the same way about his own found family, especially if he too had once been human. It's not yet confirmed in the anime if Muzan was born a human or demon, but if human, then perhaps his new family is a re-enactment of one he had in his pre-demon days.
Why Muzan Might Not Care For His Human Family
Optimistic Demon Slayer fans have compelling reasons to believe Muzan honestly loves his found family, but it's equally possible that he really is just a wolf in sheep's clothing, barely tolerating the wife and child as meals who dare to view themselves as his equals. Muzan is a clever and crafty demon, after all, and may be using these humans purely to maintain his cover so he can blend into Tokyo's society for his own reasons.
If so, that means Muzan is not only a stone-cold liar, but also oddly humble -- or at least not too proud to superficially pose as a human family man. Some anime villains would refuse to even pretend to be another race or social class based on pride alone, but not Muzan. He may be a king, but he can pretend to be a commoner if it means getting the job done. Practically comes before pride here.
What is more, Tanjiro barely spent two minutes with the family man Muzan before the villain partially broke his cover by slashing a passing man's neck to turn him into a demon, wreaking bloody havoc and allowing his family to slip away from Tanjiro in the crowd. This casual, callous disregard for human life was not only a neat trick on Muzan's part; it also served as a sign that no matter what he looks like as a family man, he is a killer above all else.
Demon Slayer wasted very little time in subverting the oddly wholesome scene of Muzan the family man, an inconclusive but still strong indicator that the demon king truly cares for no one but himself. After all, he doesn't even care for his Moons subordinates.