Complex sentence: He also aimed at introducing the Roman Rite in Paris and was progressing favorably in this direction, when he was assassinated at the church of St. Etienne du Mont by an interdicted priest named Jean-Louis Verger, who openly admitted to the crime.
Split into simpler sentences: He also aimed at introducing the Roman Rite in Paris and was progressing favorably in this direction at the time of his death. Sibour was assassinated at the church of St. Etienne du Mont by an interdicted priest named Jean-Louis Verger, who openly admitted to the crime.