It's a broadly accepted historical fact. But here are some examples:
We see it in the slaughter of the Albigensians which was really the first Roman Catholic crusade intended to slay other Christians. [K.Deschner, Opus Diaboli, Reinbek 1987, 29]The Albigensians...viewed themselves as good Christians, but would not accept Roman Catholic rule, and taxes, and dictates.
Then on the command of Pope Innocent III in 1209, Bezirs (today France) was destroyed, all the inhabitants were slaughtered. Victims including Catholics refusing to turn over their neighbors and friends were up to 70,000. [H.Wollschlger: Die bewaffneten Wallfahrten gen Jerusalem, Zrich 1973.179-181]
In Carcassonne 8/15/1209, thousands were slain and in subsequent 20 years of war until nearly all Cathars (probably half the population of the Languedoc, today southern France) were exterminated. After the war ended (1229) the Inquisition was founded 1232to search and destroy any who did not hold to the what the church dictated.
The Waldensians, and many others were exterminated, in the persecution of the Cathar of Southern Europe, northern Italy, it is estimated one million victims of the . [H.Wollschlger: Die bewaffneten Wallfahrten gen Jerusalem, Zrich 1973.183]
The Spanish Inquisition killed untold numbers and the Spanish Inquisitor Torquemada alone allegedly responsible for 10,220 executions.
John Huss, a critic of papal infallibility and indulgences, who was burned at the stake in 1415.
In the 15th century we find the Church launching a persecution against Hussites, with thousands slain. In 1538 pope Paul III declared a crusade against England who had left its domination and all English made as slaves of Church, but England being a island made that a bit difficult to implement. In the 1568, the Spanish Inquisition tribunal ordered extermination of 3 million rebels in (then Spanish) Netherlands and thousands were slain. In 1572 In France about 20,000 Huguenots were killed on command of pope Pius V. Until 17th century 200,000 flee. [K.Deschner, Opus Diaboli, Reinbek 1987, 28-31]
In the Catholic 17th century 30 years' war against the Protestants, at least 40% of population decimated, mostly in Germany. [K.Deschner, Opus Diaboli, Reinbek 1987.31-32] And its recorded in the Catholics sack the city of Magdeburg/Germany, roughly 30,000 Protestants were slain. "In a single church fifty women were found beheaded," reported poet Friedrich Schiller. [D.Stannard, American Holocaust, Oxford University Press 1992, 191.]