As population increases, animal souls are promoted to human before they are spiritually advanced enough in terms of impulse control, etc.: this shouldn't have happened, but karmic progression is automatic, after all--it's a force of nature. But the 'animalization' of humankind bodes ill for civilization.
No doubt the same goes for elves, dwarves, and so on.
Nearly every primitive culture has had rituals and celebrations to guarantee the proper passage of the seasons and to ensure the fertility of crops and animals. Oversight of these ceremonies was generally the provenance of local kings or priests.
Suppose that the adventurers dispatch one of these fellows. The local peasants may become hysterical, fearing famine and death will stalk the land. Alternatively, they may want one of the new heroes to become king. For a while, this can be a good thing, but the first time that the crops fail, the superstitious locals will want to sacrifice their new leader.