Board Thread:General Discussion/@comment-31358097-20170222030202/@comment-13682699-20170225194927

Its possible that those monsters weren't even alive (except Toriel/Gerson/Asgore) when those humans fell as they could have come way before them. However, there is dust present on two of the previous humans items (specifically the ballet and cowboy stuff) and mentions that they make the user feel more powerful/dangerous. Its implied that these humans may have killed a few monsters in self-defense or went on a no-mercy run but were take out prematurely. The only monster confirmed that they didn't kill was Toriel as we know they all made it past her in some way and she is still alive and well.

From what we know we simply cant confirm if they all went through without killing someone even accidentally out of self-defense (the only one I could honestly see doing that is the green soul), but its implied that at least 1 or 2 went neutral or no-mercy.