While Violentale is full of dark, gorey, and sad things, this is art of the Pacifist Route.
Some Pacifist Route fun details ❄️
Frisk can only achieve this by not killing any Monsters infected or otherwise, and not killing any humans, mages, or infected mages.
Papyrus becomes King of the Monsters due to his unique immunity to the debilitating aspects of the virus.
Sans and Alphys both make a serum that dampens the hunger and relaxes the mind. While they still have large appetites they are able to think clearer. It also halts the spread much like a vaccine.
Sans opens an ice cream & shaved ice place, though he does assist Alphys with her work with those infected.
Papyrus and Sans Adopt Frisk, legally Papyrus is her father but they are more like siblings with Sans taking on the more fatherly aspects.