Admitting that there's a very wide diversity of beliefs under the "Roman Catholic" banner - historic Roman Catholic armies have been eager participants in well-documented battles for the past 1,500 or so years. I'd assume that Tolkien would have had a wide variety of perfectly historic Roman Catholic arguments to chose from, to justify his fictional slaughter.
(If I recall, the orcs slaughtered in LoTR are pretty much all soldier or near-soldiers. Do orc women, children, or other non-combatants ever appear in the story?)
In many ways, that Wikipedia article feels like a Hays Code-era whitewashing of Roman Catholicism.