Undertale - you monster.....

Saving everyone except Paparyus….

The entire game kinda gives you the idea that Frisk is sorta of an avatar of your biding. He has a name and probably a history, but he's expressionless, and you don't see him talk, even tough it's obvious he's answering. So, it makes sense to mercy all, and kill one, or kill everyone and spare one, it's sorta of like a "what if i had the power to rewind everything", it gives you the chance to explore possibilities, as non sensical and sickening as they may be.