Board Thread:Questions and Answers/@comment-25094659-20160720152252/@comment-5956954-20160720152647

If you killed someone, then you need to reset and go back to the beginning.

Looking up the definition of a pacifist, it also means you have to go through the game and not kill anyone. It also requires you to befriend every main character from Papyrus to Alphys.

Hope that helps.