My Au and backstory

In Nightmare's gang, there's one major rule: NO LOVE. when Nightmare sends killer to at least get some information about the star sans's secret base, (I think I may have spelled that wrong)Killer comes back with 0.3 hp, And Night takes it into his own hands to protect him. But for some reason Nightmare's goop starts going away. Is it possible that he's feeling positivity? (Ok, that's a bit cringe but that's all I got) this is my first fanfic so please don't judge.
Warning: I have the inability to spell "sanes" correctly.