What is the purpose of humans existing in Deltarune? There is no theme of sensless war and violence between humans and monsters, so Toby could had just established Kris as a goat kid and just make it so that SOUL can enter any monster who participated in some kind of ritual.

If SOUL is specified only for humans, then that means the prophecy is either bad for monsters and good for humans or bad for humans and good for monsters. However so far the story only makes distinction between Darkners and Lightners and not really focusing on humans as separate caste different from monsters