Catholic extremists haven't stopped at non-Christians before. CW: extremely heavy including pictures Ethnic and religious persecution during WW2 by the Croatian Ustaše

Large numbers of Serbs, Jews, Gypsies, communists, and Croatians hostile to the regime were interned in concentration camps, while countless others were massacred in towns and villages. The Ustasha were directly involved in the administration and in the orchestration of the killings. In addition, some 200,000 Serbs were forcibly converted to Catholicism. According to the Ustasha blueprint, one-third of Serbs were to be killed, another third expelled, and a final third converted to Catholicism.
- "Genocides by the Oppressed: Subaltern Genocide in Theory and Practice" (2009) Ed. Nicholas A. Robins, Adam Jones

For clarity, Serbs are historically usually Eastern Orthodox Christians.