Board Thread:General Discussion/@comment-4504826-20160426040035/@comment-25638900-20160523181747

Mouwi wrote: If you do Genocide, then you cannot do Pacifist unless you install the game again. Killing Sans, Papyrus and Toriel broke my heart. Though, killing Flowey was really fun. well, you CAN do a pacifist without reinstalling, but the ending will be corrupted.

plus you can always just go into the game's folder and delete the save file