I understand why she has the gag, but why a bit gag specifically?
I think the intended joke here is that Nezuko was placed under a subtle hypnotism(?) so that she sees all humans as her family (not just her brother Tanjiro), therefore must be protected from harm. She did exactly this head-patting gesture to a human stranger in earlier episode before fighting to protect them from other demons.
In short, Nezuko here still sees Doomguy as a human being worth protecting like one of her own, whereas Doomguy himself can't bring to slay a demon this harmless and kind.
I think the intended joke here is that Nezuko was placed under a subtle hypnotism(?) so that she sees all humans as her family (not just her brother Tanjiro), therefore must be protected from harm. She did exactly this head-patting gesture to a human stranger in earlier episode before fighting to protect them from other demons.
In short, Nezuko here still sees Doomguy as a human being worth protecting like one of her own, whereas Doomguy himself can't bring to slay a demon this harmless and kind.
“Nezuko here still sees Doomguy as a human being worth protecting like one of her own” Even after godly upgrade he received?