If you finish a no mercy route and sell your soul, you can "undo" tainting a pacifist run by going into C:/Users/(YourPCNameHere)/Appdata/Local/UNDERTALE/. There will be a file called either "system_information_962" or "system_information_963" depending on how far you've gone. Delete it. WARNING: The Appdata file on some computers is HIDDEN, you will need to change your computer's settings to be able to view hidden files.