Board Thread:Questions and Answers/@comment-28847619-20160625220657/@comment-27701762-20160625222656

Thunderlight2004 wrote: Oh, okay, but I still had to start over again even though I know I didn't kill anyone. If you already started over, there is nothing that can be done. But you are in no way required to start from the very beginning. Upon defeating Flowey you can simply reload your save from before fighting Asgore and complete the remaining steps.

There is only one instance where you might not kill any character, but be required to start over from the beginning, which is if you have done something to prevent you from possibly befriending Undyne. If you do not pour water on her when reaching Hotland, then she cannot be befriended. This is, to my knowledge, the only case where the True Pacifist route is otherwise locked without killing any monsters.