Complex sentence: Unlike the Portuguese slave trade, "los Reyes Católicos" were religiously against developing that for Castile and Aragon with the slaves of Columbus, ordering many of the survivors returned to their Caribbean homelands.
Split into simpler sentences: Unlike the Portuguese Crown's support for the slave trade, "los Reyes Católicos" opposed the introduction of slavery to Castile and Aragon on religious grounds. When Columbus returned with slaves, they ordered many of the survivors to be returned to their Caribbean homelands.