"It was upon these gentle lambs, imbued by the Creator with all the qualities we have mentioned, that from the very first day they clapped eyes on them the Spanish fell like ravening wolves upon the fold, or like tigers and savage lions who have not eaten meat for days. The pattern established at the outset has remained unchanged to this day, and the Spaniards still do nothing save tear the natives to shreds, murder them and inflict upon them untold misery, suffering and distress, tormenting, harrying and persecuting them mercilessly." —Bartolomé de Las Casas

According to Bartolomé de Las Casas,

Spain needed to institute a more humane system of American Indian slavery
Spain had caused the deaths of millions of innocent people in the New World
slavery needed to be eliminated everywhere from the earth
converting American Indians to anything but Catholicism would lead to their death