"...the Spanish Republicans were hostile to the Catholic Church."

Not just a matter of abstract hatred of the Catholic Church - about 7,000 Catholic clergy, bishops, priests, seminarians and nuns, were killed by Republicans. And that leaves the laity out of the picture. (Plus a number of Basque Catholic clergy etc on the other side, killed by Franco's people.)

Civil Wars are filthy affairs. Massacres of innocent people typically get carried out by both sides, and that certainly happened in Spain.

People tend to concentrate their attention on what "the other side" does - and use it as a way to justify what "their side" does, insofar as it can't be ignored.