Board Thread:General Discussion/@comment-31358097-20170222030202/@comment-26233737-20170223204758

Yoshifan33 wrote: Maybe the monsters killed them, remember? If they did the "route" (remember, the whole thing), the monsters wouldn't have their souls. Now, also keep in mind Toriel made Sans promise not to kill another human. Another. As in, he already killed the other 6 humans who fell down. Toriel asked Sans to watch over a human if they came through the door that leads out of the Ruins. She never said the word another.

Anyways, it seems likely that the six humans either did a pacifist, or neutral routes. They only way they would be able to do a genocide is if they could reset, which we don't know if they could do or not.

Another possibility is that Undertale takes place in the aftermath of six consecutive non-geno routes. Any of the other humans may have done a genocide in a different timeline than the Undertale we know.