Whether or not there can be improprieties found in an individual Catholic in history, the Catholic Church has been a pioneer against slavery from the beginning and of mankind’s whole expedition to the New World. For example:We order and command all and each of the faithful of each sex, within the space of fifteen days of the publication of these letters in the place where they live, that they restore to their earlier liberty all and each person of either sex who were once residents of said Canary Islands, and made captives since the time of their capture, and who have been made subject to slavery. These people are to be totally and perpetually free, and are to be let go without the exaction or reception of money. If this is not done when the fifteen days have passed, they incur the sentence of excommunication by the act itself, from which they cannot be absolved, except at the point of death, even by the Holy See, or by any Spanish bishop, or by the aforementioned Ferdinand, unless they have first given freedom to these captive persons and restored their goods. We will that like sentence of excommunication be incurred by one and all who attempt to capture, sell, or subject to slavery, baptized residents of the Canary Islands, or those who are freely seeking Baptism, from which excommunication cannot be absolved except as was stated above.
(Sicut Dudum, Pope Eugene IV Against the Enslaving of Black Natives from the Canary Islands, January 13, 1435)
[T]he said Indians and all other people who may later be discovered by Christians, are by no means to be deprived of their liberty or the possession of their property, even though they be outside the faith of Jesus Christ; and that they may and should, freely and legitimately, enjoy their liberty and the possession of their property; nor should they be in any way enslaved. (Pope Paul III, Sublimus Dei, On the enslavement and evangelization of Indians, 1537)
We warn and adjure earnestly in the Lord faithful Christians of every condition that no one in the future dare to vex anyone, despoil him of his possessions, reduce to servitude, or lend aid and favour to those who give themselves up to these practices, or exercise that inhuman traffic by which the Blacks, as if they were not men but rather animals, having been brought into servitude, in no matter what way, are, without any distinction, in contempt of the rights of justice and humanity, bought, sold, and devoted sometimes to the hardest labour. (Pope Gregory XVI, In Supremo Apostolatus, Apostolic Letter condemning the slave trade, 1839)That’s just a sampling. Some of the above mention other letters of their predecessors echoing the same tradition. Click the links for more.