As today’s abortion debate illistrates:
It all depends on who gets to be defined as human!
That is a very perceptive comment. The Spanish Conquistadors had just completed the 750+ year long Reconquista in which the Iberian peninsula was retaken by various Christian kingdoms from the Muslims. The prevailing belief at the time was that the one thing that differentiated human beings from animals was the existence of a soul. The thing that justified the armed struggle against Muslims and pagans was that they had forfeited their souls rendering them not human and beyond salvation and protection by God.
Further, the practices they discovered in the New World were so extreme, even in comparison to Muslims, lead to the conclusion that the practitioners were so far beyond non-human as to be Satanic.