This was a political decision. If it concerned Sicily, that Kingdom was traditionally held to be a fief of the Papacy as a secular ruler. The Popes as secular rulers did all sorts of things including waging war. But these acts cannot be called acts of the Church.
If the Church claims some responsibility for building Western civilization
realclearreligion.org/2011/12/28/how_catholics_built_western_civilization_245366.html, it must also claim some responsibility for supporting slavery. And it has: “in the tenth century, bishops in Venice did public penance for past involvement in the Moorish slave trade and sought to prevent all Venetians from involvement in slavery.”
christianitytoday.com/ct/2003/julyweb-only/7-14-53.0.html
To the Church’s credit, her members (or other Christians) also helped end slavery as a whole in Western civilization. First, slavery among Europeans ended due to the influence of the Church and the gaining consciousness wrongness of enslaving fellow Christians. Sadly, many Christians and Catholics still felt justified in enslaving so-called pagans. Then many Christians led the way in ending the Transatlantic slave trade. For example, John Newman inspired William Wilberforce to end slavery in the British empire after almost dying and fearing Hell. And the Popes issued excommunications against slave-traders even prior to the abolishment of the Transatlantic slave trade. They largely weren’t listened to and many bishops, especially the American hierarchy, interpreted Papal condemnation of slavery in the least restrictive way possible. There has been division in the Church on related issues and between Popes. For example, Fr. Tolton was the first African-American priest in the United States and a former slave. No Catholic seminary in the United States would admit him due to the color of his skin so the Pope ordained him in Rome.
It is dishonest to deny the role of the Catholic church in slavery. The Devil is the father of lies so that will not serve the Church. The saints are designated as imperfect inspirations with Christ and Mary as perfect inspirations. Not every Pope is a saint. Consider that the first person in the Catholic Church was hurt by a so-called Christian’s sin was Jesus as he was betrayed by Peter and Judas. A great many saints were ordained despite controversy of many in the Church, St. John of the Cross was kidnapped by the Calced Carmelite priory at Toledo as a prisoner until he escaped. It was there he wrote much of his Spiritual Canticle. St. Joan of Arc was killed. St. Edith Stein wrote letters of disappointment in the Church. St. Francis of Assisi devoted his life to reform.
“Never take a man for your example in the tasks you have to perform, how ever holy he may be, for the devil will set his imperfection before you. Imitate Christ, who is supremely perfect and supremely holy, and you will never err.” - St. John of the Cross Democracy is the so called worst form of government except for all the others. Harvard has professors who plagarize and fake discoveries. The Catholic Church is full of sinners but is still the only path to moral perfection.
Stalin’s Russia was and North Korea is a slave state led by atheists. I would like to know of a non-Christian who helped end the Translatlantic slave trade. Google is the first I’ve heard of nonChristians playing a significant role in helping to end any slavery at all.