Underpain is an AU where all the previous 6 souls have harmed the monsters. Which means that the monsters have half the health than they originally had and this AU has NO RESET. And regardlessly in the end, like Flowerfell, Frisk dies. But unlike Flowerfell, Frisk dies after a few weeks on the surface with the others from an unknown sickness.

Before Frisk fell, they were an orphan and has been in the hospital for their whole life and has been taken in by the doctors and nurses there. So when they overheard that their death is just a few weeks away, they escaped the hospital and try to find an adventure of a lifetime before their death. And they did.

In here, Flowey actually tries to help you out from the surface even though, he tried to kill you several times.

Toriel, was harmed by Intergrity.
Undyne was harmed by Kindness, hence the burned mark on her remaining eye.
Alphys was scratched and shot by Justice. Mettaton helps guide her around but she is able to do experiments with her other senses.
Sans was harmed by Patience's stab in the eye. His magic isn't working so well, it's weak.
Papyrus was harmed by Bravery, hence the dents and crack.
Asgore was harmed by Perseverance. Though you can't see it, he has scratch marks.

So uh, they manage to survive the attacks. And then, there's you. Sure you can commit genocide, but in here, Chara's long gone. The only reason why the monsters still lives from the attacks is because Chara watches over them and makes sure the souls who harmed them are killed. So if you commit genocide, you can't, because Chara is right next to you, behind you, tracking your every move. Plus, why would you commit genocide anyway?

Wait....don't answer that